I know this used to be a free tool, but the devils at redgate are now charging for
Reflector[
^]. Maybe you can still find an old version out there and see if still fits your needs. With Reflector you can disassemble a .NET dll or exe back to source code. So for your translation process, let's say VB.NET -> C#, you would compile the VB.NET project and then disassemble that into C#. For C# -> VB.NET you'd first build the C# project and then disassebmle that into VB.Net Done!
Caveat: The link I posted in this solution points to a commercial product. I don't advise you to use it, as I abhor people making money out of former open/free solutions. If you need this professionally it might still be a viable solution for you. Especially since smallest available version would already be fit for your needs.
Best regards,
—MRB