Adding vBulletin to an Active Members Site

[l3vi]

I got a client that owns this site, they want to add a forum to the site. At this point they have something around 30,000 members, so my options are to write one from scratch (ugh!) or try to install vBulletin. The thing I am wanting to know is what would I be looking at with keeping users from having to double register?

They want all members of the site to have access to all the things you do on the forums, but they dont want them to have to register, reset passwords, etc from vBulletin. It has to happen all from their side of the site, because they do not want any problems related with confusion of dual logins, etc.

I have been thinking another option would be to change their software to use vBs user database to control logins, etc (Don't know, could be risky).

I appreciate any input.

[R1lover]

What software is that site running now?

[l3vi]

The site's software is custom built.