Have you read up on the common language runtime? If not, the bottom line is that yes, you can use c# classes with vb.net classes. In fact, you can use any .net language with any other .net language. I suppose there's more to the CLR than that, but that's what affects us programmers earning our daily bread.
In order to use two different .net languages together, you must compile them into their own assemblies. For instance, say you had these assemblies:
MySolution.ExtendedUI.vb //vb.net file
MySolution.ShoppingCart.cs //c# file
After Compiling...
MySolution.ExtendedUI.dll
MySolution.ShoppingCart.dll
At this point, your further programming doesn't really care what languages the assemblies are written in. You could use these assemblies with JScript.net if you wanted to. If you used ExtendedUI in Shoppingcard, no problems either.
If you're not familiar with compiling dll's, post and I'll try to find a link or something for you.