I think - and i'm not at my music PC to check - that vstsynthfont64 won't work as a vst plugin like say izotope nectar, but will work if you add it as a synth.

in RB you can't add it as a plugin on a track using the fx drop down but if you right click on the track and choose the option to use a specific midi synth (or words to that effect) for the track it can be added.

i don't use various synths in BIAB so i'm not sure but i think the reason you can't add it as a vst like a normal plugin is probably the same