Not 'quite' sure what you're describing, but in VS2005, a VB.NET project can only use VB.NET code.
Now, you can take a C# class, compile it to a DLL, and then add a reference to it in a VB.NET project. But I'm pretty certain that vs2005 won't recognize both VB.NET and C# code in the same project.
Does that help?
Kevin