VB.NET and C#.NET code is very similar in every context except the actual language syntax. In my experience, learning .NET is more about learning the framework itself, so I wouldn't be concerned about working in Visual Basic rather than C# - if you have worked with one, you can pick up the other in a very short time.
Knowing .NET a valuable skill set to have and should be for some time to come. A few years of solid experience using it should certainly lead to greater opportunities in the future.
I can't speak as much for ADO.NET's widespread usage; my company still uses it exclusively. I think it will be awhile before LINQ and other newer technologies muscle it out completely, though.
If it's an interesting place to work, you should't let the exclusive use of VB.NET and ADO.NET hold you back from accepting a job there, IMO.