Learning C# would be good place to start. (Unless there's VB bindings...?)
I've not touched VB.NET more than I've had to, but... all .NET languages should be able to use all .NET assemblies (DLLs) without needing special bindings or anything. Although there are some CLR features VB.NET doesn't support (hell, there are some CLR features C# doesn't support) so it's theoretically possible to write an API that you can't properly use with VB.NET.
I seem to recall that MS only offers Game Studio Express in C# flavour, which is more of a stumbling block. But I expect it's possible with only a little fiddling to get VB.NET XNA apps compiled.
The real question is why you'd want to. Leaving aside the VB-bashing, all the examples, tutorials and so on you'll find will be in C#, not to mention that it's a more useful skill for the CV.