I suppose if you stipulated in the "competition" rules that the registrations would only count if the registered user made 5 (or more) posts (over a period of 30 days) which positively contributed to the conversation here, then they would count.
You could integrate into vB a system which allows members to create an "affiliate" link (like "http://equinox.signmeup.hostingdiscussion.com/") and puts that in the "Referrer" block in the registration form.
After a given period, a SQL statement could pick out all of the referees who have "contributed" and then...ooohh...I love sounding like a programmer ;P
Anyway, you get the idea...could work...
Edit: No idea why I didn't see the edit button on the top post. Sorry for the double post