Focus on C#. But at least be able to read and understand VB.NET code.

Now, for a long time, it could be argued that VB.NET lagged behind C#. But I personally don't think that'll be true from now on. But regardless, my two cents is to focus on C#. Most of the developer projects I've worked on, have been in C#.

About 98% of the code I've written has been in C#. But I've had to write a few VB.NET classes and I've had to modify a little bit of VB.NET code.