I set up the piece in biab first, then I load the vst in reaper, change the settings to direct the stems to the folder I work from (so the stems get saved at that location) and then I load the song in the vst. So far so good, works fine every time,and I can then work on the arrangement / mixing in reaper. This is for the legacy tracks. The utility tracks do not load. but i can keep regenerating the legacy tracks / adding markers/ chords... It has got limitations, it doesn't work as biab, but I get what I need from it. Thank you all for the feedback.

Artur