I assume you are using XAML and this achievable through styles
Add these styles:
<!-- Simple Splitter Style -->
<Style x:Key="SimpleSplitterStyle" TargetType="controls:GridSplitter">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="controls:GridSplitter">
<Grid x:Name="Root" IsHitTestVisible="{TemplateBinding IsEnabled}">
<Rectangle Fill="{TemplateBinding Background}" StrokeThickness="0"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<!-- Tab Item Header (Right) -->
<Style x:Key="RightTabItemHeader" TargetType="ContentControl">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ContentControl">
<ContentPresenter Cursor="{TemplateBinding Cursor}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" ContentTemplate="{TemplateBinding ContentTemplate}">
<layout:LayoutTransformer >
<layout:LayoutTransformer.LayoutTransform>
<RotateTransform Angle="90"/>
</layout:LayoutTransformer.LayoutTransform>
<ContentPresenter Margin="5,0,5,0" />
</layout:LayoutTransformer>
</ContentPresenter>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<!-- Tab Item Header (Left) -->
<Style x:Key="LeftTabItemHeader" TargetType="ContentControl">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ContentControl">
<ContentPresenter Cursor="{TemplateBinding Cursor}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" ContentTemplate="{TemplateBinding ContentTemplate}">
<layout:LayoutTransformer >
<layout:LayoutTransformer.LayoutTransform>
<RotateTransform Angle="-90"/>
</layout:LayoutTransformer.LayoutTransform>
<ContentPresenter Margin="5,0,5,0" />
</layout:LayoutTransformer>
</ContentPresenter>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<!-- Tab Item Style -->
<Style x:Key="TabItemStyle" TargetType="controls:TabItem">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="controls:TabItem">
<Grid x:Name="Root">
<Grid x:Name="TemplateTopSelected" Visibility="Collapsed" Canvas.ZIndex="1">
<Border Margin="-2,-2,-2,0" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="1,1,1,0" CornerRadius="3,3,0,0">
<Border BorderBrush="#FFFFFFFF" BorderThickness="1" CornerRadius="1,1,0,0">
<Border.Background>
<LinearGradientBrush EndPoint=".7,1" StartPoint=".7,0">
<GradientStop Color="#FFFFFFFF" Offset="0"/>
<GradientStop Color="#F9FFFFFF" Offset="0.375"/>
<GradientStop Color="#E5FFFFFF" Offset="0.625"/>
<GradientStop Color="#C6FFFFFF" Offset="1"/>
</LinearGradientBrush>
</Border.Background>
<Grid>
<Rectangle Fill="#FFFFFFFF" Margin="0,0,0,-2"/>
<ContentControl x:Name="HeaderTopSelected" FontSize="{TemplateBinding FontSize}" Foreground="{TemplateBinding Foreground}" IsTabStop="False" Cursor="{TemplateBinding Cursor}" HorizontalAlignment="{TemplateBinding HorizontalAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalAlignment}"/>
</Grid>
</Border>
</Border>
<Border x:Name="FocusVisualTop" Margin="-2,-2,-2,0" IsHitTestVisible="false" Visibility="Collapsed" BorderBrush="#FF6DBDD1" BorderThickness="1,1,1,0" CornerRadius="3,3,0,0"/>
<Border x:Name="DisabledVisualTopSelected" Margin="-2,-2,-2,0" IsHitTestVisible="false" Opacity="0" Background="#8CFFFFFF" CornerRadius="3,3,0,0"/>
</Grid>
<Grid x:Name="TemplateTopUnselected" Visibility="Collapsed">
<Border x:Name="BorderTop" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="1" CornerRadius="3,3,0,0">
<Border x:Name="GradientTop" BorderBrush="#FFFFFFFF" BorderThickness="1" CornerRadius="1,1,0,0">
<Border.Background>
<LinearGradientBrush EndPoint=".7,1" StartPoint=".7,0">
<GradientStop Color="#FFFFFFFF" Offset="0"/>
<GradientStop Color="#F9FFFFFF" Offset="0.375"/>
<GradientStop Color="#E5FFFFFF" Offset="0.625"/>
<GradientStop Color="#C6FFFFFF" Offset="1"/>
</LinearGradientBrush>
</Border.Background>
<Grid>
<ContentControl x:Name="HeaderTopUnselected" FontSize="{TemplateBinding FontSize}" Foreground="{TemplateBinding Foreground}" IsTabStop="False" Cursor="{TemplateBinding Cursor}" HorizontalAlignment="{TemplateBinding HorizontalAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalAlignment}"/>
</Grid>
</Border>
</Border>
<Border x:Name="DisabledVisualTopUnSelected" IsHitTestVisible="false" Opacity="0" Background="#8CFFFFFF" CornerRadius="3,3,0,0"/>
</Grid>
<Grid x:Name="TemplateBottomSelected" Visibility="Collapsed" Canvas.ZIndex="1">
<Border Margin="-2,0,-2,-2" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="1,0,1,1" CornerRadius="0,0,3,3">
<Border BorderBrush="#FFFFFFFF" BorderThickness="1" CornerRadius="0,0,1,1">
<Border.Background>
<LinearGradientBrush EndPoint=".7,1" StartPoint=".7,0">
<GradientStop Color="#FFFFFFFF" Offset="0"/>
<GradientStop Color="#F9FFFFFF" Offset="0.375"/>
<GradientStop Color="#E5FFFFFF" Offset="0.625"/>
<GradientStop Color="#C6FFFFFF" Offset="1"/>
</LinearGradientBrush>
</Border.Background>
<Grid>
<Rectangle Fill="#FFFFFFFF" Margin="0,-2,0,0"/>
<ContentControl x:Name="HeaderBottomSelected" FontSize="{TemplateBinding FontSize}" Foreground="{TemplateBinding Foreground}" IsTabStop="False" Cursor="{TemplateBinding Cursor}" HorizontalAlignment="{TemplateBinding HorizontalAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalAlignment}"/>
</Grid>
</Border>
</Border>
<Border x:Name="FocusVisualBottom" Margin="-2,0,-2,-2" IsHitTestVisible="false" Visibility="Collapsed" BorderBrush="#FF6DBDD1" BorderThickness="1,0,1,1" CornerRadius="0,0,3,3"/>
<Border x:Name="DisabledVisualBottomSelected" Margin="-2,0,-2,-2" IsHitTestVisible="false" Opacity="0" Background="#8CFFFFFF" CornerRadius="0,0,3,3"/>
</Grid>
<Grid x:Name="TemplateBottomUnselected" Visibility="Collapsed">
<Border x:Name="BorderBottom" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="1" CornerRadius="0,0,3,3">
<Border x:Name="GradientBottom" BorderBrush="#FFFFFFFF" BorderThickness="1" CornerRadius="0,0,1,1">
<Border.Background>
<LinearGradientBrush EndPoint=".7,1" StartPoint=".7,0">
<GradientStop Color="#FFFFFFFF" Offset="0"/>
<GradientStop Color="#F9FFFFFF" Offset="0.375"/>
<GradientStop Color="#E5FFFFFF" Offset="0.625"/>
<GradientStop Color="#C6FFFFFF" Offset="1"/>
</LinearGradientBrush>
</Border.Background>
<Grid>
<ContentControl x:Name="HeaderBottomUnselected" FontSize="{TemplateBinding FontSize}" Foreground="{TemplateBinding Foreground}" IsTabStop="False" Cursor="{TemplateBinding Cursor}" HorizontalAlignment="{TemplateBinding HorizontalAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalAlignment}"/>
</Grid>
</Border>
</Border>
<Border x:Name="DisabledVisualBottomUnSelected" IsHitTestVisible="false" Opacity="0" Background="#8CFFFFFF" CornerRadius="0,0,3,3"/>
</Grid>
<Grid x:Name="TemplateLeftSelected" Visibility="Collapsed" Canvas.ZIndex="1">
<Border Margin="-2,-2,0,-2" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="1,1,0,1" CornerRadius="3,0,0,3">
<Border BorderBrush="#FFFFFFFF" BorderThickness="1" CornerRadius="1,0,0,1">
<Border.Background>
<LinearGradientBrush EndPoint=".7,1" StartPoint=".7,0">
<GradientStop Color="#FFFFFFFF" Offset="0"/>
<GradientStop Color="#F9FFFFFF" Offset="0.375"/>
<GradientStop Color="#E5FFFFFF" Offset="0.625"/>
<GradientStop Color="#C6FFFFFF" Offset="1"/>
</LinearGradientBrush>
</Border.Background>
<Grid>
<Rectangle Fill="#FFFFFFFF" Margin="0,0,-2,0"/>
<ContentControl x:Name="HeaderLeftSelected" FontSize="{TemplateBinding FontSize}" Foreground="{TemplateBinding Foreground}" IsTabStop="False" Cursor="{TemplateBinding Cursor}" HorizontalAlignment="{TemplateBinding HorizontalAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalAlignment}" Style="{StaticResource LeftTabItemHeader}"/>
</Grid>
</Border>
</Border>
<Border x:Name="FocusVisualLeft" Margin="-2,-2,0,-2" IsHitTestVisible="false" Visibility="Collapsed" BorderBrush="#FF6DBDD1" BorderThickness="1,1,0,1" CornerRadius="3,0,0,3"/>
<Border x:Name="DisabledVisualLeftSelected" Margin="-2,-2,0,-2" IsHitTestVisible="false" Opacity="0" Background="#8CFFFFFF" CornerRadius="3,0,0,3"/>
</Grid>
<Grid x:Name="TemplateLeftUnselected" Visibility="Collapsed">
<Border x:Name="BorderLeft" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" CornerRadius="3,0,0,3">
<Border x:Name="GradientLeft" BorderBrush="#FFFFFFFF" BorderThickness="1" CornerRadius="1,0,0,1">
<Border.Background>
<LinearGradientBrush EndPoint=".7,1" StartPoint=".7,0">
<GradientStop Color="#FFFFFFFF" Offset="0"/>
<GradientStop Color="#F9FFFFFF" Offset="0.375"/>
<GradientStop Color="#E5FFFFFF" Offset="0.625"/>
<GradientStop Color="#C6FFFFFF" Offset="1"/>
</LinearGradientBrush>
</Border.Background>
<Grid>
<ContentControl x:Name="HeaderLeftUnselected" FontSize="{TemplateBinding FontSize}" Foreground="{TemplateBinding Foreground}" IsTabStop="False" Cursor="{TemplateBinding Cursor}" HorizontalAlignment="{TemplateBinding HorizontalAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalAlignment}" Style="{StaticResource LeftTabItemHeader}"/>
</Grid>
</Border>
</Border>
<Border x:Name="DisabledVisualLeftUnSelected" IsHitTestVisible="false" Opacity="0" Background="#8CFFFFFF" CornerRadius="3,0,0,3"/>
</Grid>
<Grid x:Name="TemplateRightSelected" Visibility="Collapsed" Canvas.ZIndex="1">
<Border Margin="0,-2,-2,-2" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="0,1,1,1" CornerRadius="0,3,3,0">
<Border BorderBrush="#FFFFFFFF" BorderThickness="1" CornerRadius="0,1,1,0">
<Border.Background>
<LinearGradientBrush EndPoint=".7,1" StartPoint=".7,0">
<GradientStop Color="#FFFFFFFF" Offset="0"/>
<GradientStop Color="#F9FFFFFF" Offset="0.375"/>
<GradientStop Color="#E5FFFFFF" Offset="0.625"/>
<GradientStop Color="#C6FFFFFF" Offset="1"/>
</LinearGradientBrush>
</Border.Background>
<Grid>
<Rectangle Fill="#FFFFFFFF" Margin="-2,0,0,0"/>
<ContentControl x:Name="HeaderRightSelected" FontSize="{TemplateBinding FontSize}" Foreground="{TemplateBinding Foreground}" IsTabStop="False" Cursor="{TemplateBinding Cursor}" HorizontalAlignment="{TemplateBinding HorizontalAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalAlignment}" Style="{StaticResource RightTabItemHeader}"/>
</Grid>
</Border>
</Border>
<Border x:Name="FocusVisualRight" Margin="0,-2,-2,-2" IsHitTestVisible="false" Visibility="Collapsed" BorderBrush="#FF6DBDD1" BorderThickness="0,1,1,1" CornerRadius="0,3,3,0"/>
<Border x:Name="DisabledVisualRightSelected" Margin="0,-2,-2,-2" IsHitTestVisible="false" Opacity="0" Background="#8CFFFFFF" CornerRadius="0,3,3,0"/>
</Grid>
<Grid x:Name="TemplateRightUnselected" Visibility="Collapsed">
<Border x:Name="BorderRight" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="1" CornerRadius="0,3,3,0">
<Border x:Name="GradientRight" BorderBrush="#FFFFFFFF" BorderThickness="1" CornerRadius="0,1,1,0">
<Border.Background>
<LinearGradientBrush EndPoint=".7,1" StartPoint=".7,0">
<GradientStop Color="#FFFFFFFF" Offset="0"/>
<GradientStop Color="#F9FFFFFF" Offset="0.375"/>
<GradientStop Color="#E5FFFFFF" Offset="0.625"/>
<GradientStop Color="#C6FFFFFF" Offset="1"/>
</LinearGradientBrush>
</Border.Background>
<Grid>
<ContentControl x:Name="HeaderRightUnselected" FontSize="{TemplateBinding FontSize}" Foreground="{TemplateBinding Foreground}" IsTabStop="False" Cursor="{TemplateBinding Cursor}" HorizontalAlignment="{TemplateBinding HorizontalAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalAlignment}" Style="{StaticResource RightTabItemHeader}"/>
</Grid>
</Border>
</Border>
<Border x:Name="DisabledVisualRightUnSelected" IsHitTestVisible="false" Opacity="0" Background="#8CFFFFFF" CornerRadius="0,3,3,0"/>
</Grid>
<Border x:Name="FocusVisualElement" Margin="-1" IsHitTestVisible="false" Visibility="Collapsed" BorderBrush="#FF6DBDD1" BorderThickness="1" CornerRadius="3,3,0,0"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
And Then you can assign this to each tab item:
<controls:TabControl TabStripPlacement="Right" Margin="4">
<controls:TabItem Header="First" Style="{StaticResource TabItemStyle}">
<controls:TreeView>
<controls:TreeViewItem Header="Parent 1" />
<controls:TreeViewItem Header="Parent 2">
<controls:TreeViewItem Header="Child 1">
<controls:TreeViewItem Header="Grandchild 1" />
</controls:TreeViewItem>
</controls:TreeViewItem>
<controls:TreeViewItem Header="Parent 3">
<controls:TreeViewItem Header="Child 1">
<controls:TreeViewItem Header="Grandchild 1" />
<controls:TreeViewItem Header="Grandchild 2" />
</controls:TreeViewItem>
</controls:TreeViewItem>
</controls:TreeView>
</controls:TabItem>
<controls:TabItem Header="Second" Style="{StaticResource TabItemStyle}">
<controls:TreeView>
<controls:TreeViewItem Header="Parent 1" />
<controls:TreeViewItem Header="Parent 2">
<controls:TreeViewItem Header="Child 1">
<controls:TreeViewItem Header="Grandchild 1" />
</controls:TreeViewItem>
</controls:TreeViewItem>
<controls:TreeViewItem Header="Parent 3">
<controls:TreeViewItem Header="Child 1">
<controls:TreeViewItem Header="Grandchild 1" />
<controls:TreeViewItem Header="Grandchild 2" />
</controls:TreeViewItem>
</controls:TreeViewItem>
<controls:TreeViewItem Header="Parent 4">
<controls:TreeViewItem Header="Child 1">
<controls:TreeViewItem Header="Grandchild 1" />
<controls:TreeViewItem Header="Grandchild 2" />
<controls:TreeViewItem Header="Grandchild 3" />
<controls:TreeViewItem Header="Grandchild 4" />
<controls:TreeViewItem Header="Grandchild 5" />
<controls:TreeViewItem Header="Grandchild 6" />
<controls:TreeViewItem Header="Grandchild 7" />
<controls:TreeViewItem Header="Grandchild 8" />
</controls:TreeViewItem>
</controls:TreeViewItem>
</controls:TreeView>
</controls:TabItem>
</controls:TabControl>
Now you can Change from Top/Bottom/Right/Left and the text will go vertical on Right/Left. I left top and bottom alone in this example but this should give you plenty of ideas on how to control these tabs.