I'm no vbulletin expert, but it is written in php, which is that smart. Might want to add a spam detection hook to the message body on save/update to notify the mods, assuming such a hook doesn't already exist (or get the vbulletin folks to do it).
Edit: I see they have things like auto moderation by keyword
http://www.vbulletin.org/forum/showt...hlight=spammer
so that a mod would have to approve such a message first.