vBulletin has their own referral code as far as i am aware.
http://your.path.to/forum/register.php?referrerid=X
Where X is the referrers userid. Basically your members can post links like this for other people to join and automatically have themselves as the referrer. new users can still remove the name, but the link automatically fills it in.
WHATEVER way that a new user joins and indicates their refererrer (by link, typing it in, etc) then the referrer will get credits. you can also verify new referrals by waiting until they are a certain number of days registered, or reach a certain postcount and/or reputation, before they get the credits for the referral