VB.Net shares the same compiler/JIT/Runtime technology as C#, and they've had a decree of maintaining feature-parity for awhile now. You can basically think of them as two dialects that speak differently but otherwise have the identical capacity for communicating powerfully. C# is neither faster, nor slower than VB.net in mainstream terms -- I suppose its possible that the patterns one or the other encourages might be marginally slower or faster in specific cases, but you should mostly be free to defy those patterns. I don't hesitate at all to say that, between the two, use what's most comfortable because neither has a clear performance or expressive advantage. I happen to think that C# is the better language, myself, and probably a significant majority would agree, but that's no good reason to dump VB.net if you're already familiar with it.
That said, there are ecosystem differences -- as I said, most people seem to prefer C# by a significant margin, and so its easier to find people to help you out if you're writing your code in C#. Likewise, there tend to be more tutorials and references written in C#, and also some of the VB.Net code you'll find are straight-line ports of the C# version, which may not yield idiomatic VB.net. Moreover, most .net libraries are written in C#, so you'll eventually want to be comfortable at least reading it. All of those things combined tend to mean that any VB.net programmer who sticks with programming long enough eventually will also pick up C# (except, perhaps, those whose job is solely to maintain large, legacy VB code bases, as sometimes exist in enterprises).
If you were new, and asking the question "Should I learn C# or VB.Net" I would hands-down recommend C#, but since you are coming already with VB.Net experience, there's no need for you to drop what you're doing now to take a break and learn C# before returning to your project. You will probably (want to) come to learn C# before long, and that's a good thing, but let that come to you as you need it is my advice, unless you are simply itching to learn a new language, or are finding that not knowing C# is holding you back right now.