I have noticed a pretty large increase in spam here, I assume that the forum have gotten indexed and it's bots that search for standardized forums (vBulletin in this case) and makes spam bots on it...
Can't remember but do the forum require email activation and does it have captcha for registration? These are probably the two features that would reduce these bots (a lot of stupid pointless boring work to clean out the forum from spam bots)