I have just enabled logging in to the forums using Facebook connect. The feature is new to vbulletin and has a bug when existing users try to use it.
New registrations work OK.
If you are an existing user try this to get it to work
- Login using the regular login forum process, not Facebook Connect
- Go to the connections tab in your user profile settings.
- Activate Facebook connect using your Facebook email address and password
- Log out and then login using Facebook Connect and your Facebook email address and password.
Let me know if it works for you.