Silverlight/Prism injecting stackpanel into view from viewModel

Question

I have a StackPanel in my View:

<StackPanel x:Name="VariablesPanel"></StackPanel>

And I want to add into this StackPanel other StackPanel which I build in ViewModel.

this._unityContainer.Resolve<DeductionView>().VariablesPanel.Children.Add(allStackPanel);

and it doesn't work... I have no idea why... If i use this method in codebehind everything seems to be ok, but doesn't work in ViewModel.