yes

Yes you can add C# and VB.Net code in the same asssembly. 
For that add a new item to the project. 
Give the name of the file as Class1.vb if you want to add a 
vb code and Class1.cs if you want to add C# code. 

When you compile the project it will compile to a single 
assembly. 
The language does not matter in the end since everthing 
will be compile to MSIL.

Yes, as Antony said, we can do the same but VB files will 
not be available in any C# application where we are using 
that class assembly.

Yes We can add same both code one Assembly .Bcoz Shared 
Assembly is Same Application