The vBulletin user integration module will add existing vBulletin users to a new Pligg CMS website. It works by adding vBulletin users into the Pligg user database either when the user signs in using their vBulletin credentials or if the user is already signed it it will use their vBulletin cookie to automatically log them in to Pligg CMS.
This module is intended to replace the Pligg user registration process, since registrations will be handled by vBulletin. You should make sure that all links pointing to Pligg’s registration page are updated to point to vBulletin’s. You should also edit the register_center.tpl file so that it forwards to the vBulletin registration page, so that users don’t create accounts through Pligg CMS. It may even be a good idea to remove or rename Pligg’s register.php file.