I am not an MP3Tag developer, but I think I can answer some of your questions.
MP3 tag editing is done via ID3Lib. For most of the other tag formats, Florian uses the native SDK as far as I know (FLAC --> FLAC Meta Data Interface for example).
Anyways, he might correct me if I am mistaken, but he will be back from holidays in about two weeks.
PS: You can use the API in VB if the libraries' (ID3Lib, etc.) functions are exported with _stdcall. For _cdecl, you have to use a C wrapper.