Can I link a visual basic application to C#?

Yes, provided the apps are all written for .NET (i.e. the VB code is after V6.0).

.NET applications all produce .NET Assembly files, which contain intermediate code that can be accessed from any .NET language and used as if it was in the same language.
All you have to do is add the DLL or EXE assembly files as references to your C# project and you will have access to the VB forms as if they were written in C# orginaly.

Do note that not all EXE or DLL files are .NET assemblies: these extensions were also used for their Native code equivalents, which cannot be referenced at all but must be accessed via DLLImport, a much more complex approach.