vBulletin can definitely be set to prevent links from new members, and the number of necessary posts can be set by the admins. I've seen anywhere from 5 to 50. It can also set moderator approval of new members. This keeps spammers from posting random messages to make their count. Things like "Hi, how are you?" and nothing else.
Some spammers will take lines or entire posts from the site to make it look relevant. I saw one once that made it clear that it was random. They'd copied one that was fairly recent and had quite memorable details, something that was highly unlikely to be duplicated by an actual new user. Something like "We were supposed to go on vacation to Disneyland [more details about why this choice and how the kids were all excited] and then my husband [details about husband doing handyman work] fell off the roof and broke both legs. And then my MIL said since he couldn't go anyway she should go in his place and I said not only no but heck no!"