MAKE ALL YOUR USERS REGISTER IN PHPBB3 WITH THE SAME USERNAME AND PASSWORD
You can try deleting any usergroup entries in the photopost usergroups table and photopost users table. You can then rerun install.php to integrate with phpbb and install that integration correctly. You would then run this script to update your photos and comments usernames with the proper new userid from phpbb3
Reassign userid's when switching integrations to VBulletin