My opinion would also be to stick with C# as VB.net would not leverage your skills in any way (my personal opinion and I do not state that this is indeed a generalised fact).
I've been programming for 6 years in C# and VB.net and I always prefer doing C# over VB.net, C# just feels more natural and the syntax is spot on where VB just does not feel right.
But at the end of the day,the more languages you know the better,but I would not more money to get certification for VB.net....C# is all you need.If you do have some spare time and want to start doing VB.net then go for it.