If you have the backglass showing in pinballx, with autopositionb2s=False, when you open a table can you ALT+TAB to see the hidden backglass in game? And what happens if you load again directly outside pinballx?
I think there's a mismatch between screen numbers for pinballx & VPX/B2S (set in screenres.txt file).. you may need to change screenres.txt to show b2s in game by relative position rather than by screen number, info on that in this post https://www.vpforums.org/index.php?showtopic=38775#entry406938