Vbulletin and Phpizabi will be installed on the same domain.
The usernames and passwords should be shared between the scripts.
Ideally, the user can log in at either one of the scripts and be logged in at the other.
Or the vbulletin log in could be integrated into Phpizabi.
When a user registers at the vbulletin forum, a profile should be created for this user at phpizabi and vice versa.
Additionally, a small script is needed to initially port the existing vbulletin users to phpizabi profiles.