Using our own user database

[Chroder]

There are a couple ways you can do this.

First is to hack the way one of the softwares (be it your existing form or vBulletin itself) to use the other user base for user-related things. vBulletin is complex so I wouldn't recommend hacking the way it handles users, but if your existing app only stores users and passwords and thats about it, then perhaps you can make it use the vBulletin user base. If this were the case you could simply create a script to copy users into vBulletin, then edit the other app to start using the vB user base.

The second way is to keep the two user bases synced. So when a new user signs up in one app, then automatically create a record in the other app. Downside to this is that you also have to create code in both applications that will update the other data whenever there is an update. For example, user changes a password in vB you need to update the password in the other app as well.

Unfortunately there's no easy way to integrate the vB user base with other applications. Something like LDAP is what many vB customers want very badly. So when you ask 'how', it's not really that simple. You will need to know how to code, or hire a coder to do the job for you.