Well this is what I would do.
1. Use vbulletin impex to import photopost users. (THEY NEED TO SUPPORT THEIR SOFTWARE NOT US
2. Backup your photopost database. Reinstall Photopost to integrate with vbulletin. Make sure before you do that to clear the Photopost users table and usergroups tables in your photopost database of any entries before you begin.
3. After Photopost is successfully integrated with vb
use this script I wrote to update the users userids since the vb
impex script will change them. Then you should be fine. Reassign userid's when switching integrations to VBulletin