I am also converting Vbulletin 4.2.0 over to BBPress 2.3-rc1 (not 2.2.3) and hopefully BuddyPress. I had the same problem with users not coming over.
I found that if I disabled BuddyPress, and ran the conversion, my users came over with no problem.
Now that I’m enabling Buddypress and have a few glitches, such as the members page not displaying members, and the registration page not displaying.
