I admin several forums, but none of them are vbulletin. One thing that I've seen that is pretty common is having a necessary number of posts before you can post links. I've noticed several people have posted links in their topics and it was their first post. These posts generally are to drive traffic to another site, be it legitimate or not.
Just a thought.
Also, if you need any forum admin/moderator help, feel free to hit me up.