C# is not better than VB.Net, they both run on top of the same .Net stack. The rest is just syntax candy. Saying that I can write in C# or VB.Net and generally I write in C# as I prefer the syntax. In the end its a job and the best choice is where the market demand is and that is generally in C#. Why not learn C#, if you know VB.Net you need to learn the syntax but you already know the framework, it's what I did.

If they are that rude on a job posting I wouldn't touch them with a barge pole.