[davide101]

To create a plugin for two vBulletin forums would take about 15 hours of work ($750).

(Required) vBulletin vBSSO minimal package implementation
1. Setup package - 1h
1. Handle login event - 1h
2. Handle logout event - 1h
3. Handle register event - 2h
4. Sync user credentials if user changes his email in vBulletin - 1h
5. vBSSO Configuration page - 1h
6. Adapt primary vBSSO Access Settings page to filter that vBulletin could be like a connected platform - 3h
7. Testing, packaging, and deploying - 5h

(Optional) vBulletin vBSSO optional implementation
1. Override login, logout, register creating new custom vBSSO block in vBulletin as custom solution, because vBulletin user a form to submit user login data by default., vB has no API to overwrite login block itself, but allows to create new login page. 1-2hr

Account Collisions
If you have two different sites, this would not solve the issue with the issue of two users having the same username or email addresses on multiple sites. If there is serious interest in this, we could work out a system that handles those duplicates. For example, we could offer the ability to change the username to something that doesn't exist or to allow the same username if the email address matches.