No one is exploiting vulnerabilities in vBulletin, not all spammers are BOTS, not by a long shot. There are people paid to join forums, there are people who do it themselves and make a business out of it. With Google they can answer just about any question you could ever think of.
This will only become more common in the future as well.
There's no inherent flaw in Q&A human verification other than humans can get in.
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