ILSpy can be used as the open-source desktop code converter:
The best thing is that you won't be dependent on any sites or services. This software has a lot more uses; this is an open-source replacement for the famous Reflector:
This is how to use it for translation: having some C# code, compile it into any assembly. Load this assembly with ILSpy from a file (such as .DLL, .EXE…), locate the code you are interested in. For output language, select VB.NET. When you disassemble the code, it will be shown in VB.NET. Same thing to get C# from VB.NET.