C# is not a bridge between VB and C++. If you want to write managed C++ code then you would use C++/CLI, which is basically C++.NET. C# is basically just an alternative to VB.NET with basically all the same features but a C-based syntax instead of a VB-based syntax.
It actually may be the case that you could replace your unmanaged code with C#. The one genuine advantage that C# has over VB.NET is support for pointers. As such, you may be able to write your string manipulation code in C# and then either reference that assembly in a VB.NET app ro simply write the whole app in C#.