Will,
Midi can be tricky, but here is my take.
Sampletank is a complex software "sound module", I do not believe it is distributed by PG.

Regardless of that, If you have a track that is MIDI, you can go to "plugin" tab on mixer and choose the VST you want to use to replace the basic sounds of BIAB midi.

If you are new at midi, I would recommend experimenting with single instrument VST. Here is a very good (free) guitar:
https://www.amplesound.net/en/pro-pd.asp?id=7


Some VST allow for bank/sound saves. Not sure if BIAB will load particular sound bank that you selected in VST before closing the project (save state I believe is called FXB). If I need to save sounds and be sure they will sound same way they did, I save them within the VST and recall the saved state for particular song.

For example You save your song in BIAB as Song1 so you can save settings for VST (in VST) and save it as Song1 so you know which settings relate to which song.

I hope I am making sense here smile