Is it possible to call a function written in c# from a class written in VB or vice versa?

Yes of course. That's the whole idea behind .NET. The easiest way is to write your code and compile them to Class Libraries. You can then consume this library from your client code. For example, you can write code in VB.NET and compile it into a Class Library. You can then write a Windows application in C# and use the functions from the Class Library in your windows app.

Yes. Put your code into a DLL and have them reference it (or vice versa).

This was a main reason I refused to take a job somewhere. The main 'senior' programmers used VB and knew I was a C# programmer. They asked me if I would be OK with not using C# anymore.
I asked a few follow up questions and they clearly did not understand what .Net really was

It might helps,

How to manage a multilanguage component solution in Visual C#[^]

:)

Yes it is. I made a C# .NET Class library which was COM Visible that was used in both VS6 C++ and a VS6 VB program. So there is a possibility of going to something that doesn't use the .NET framework too!