Set up a vBulletin installation, remove all Anti-bot measures (Verification questions, CAPTCHA, Email activation etc...) and watch them come rolling in. I accidentally did this with a phpBB3 Installation that I forgot about (At the time, all anti-spam measures were either non-existent or disabled by default). Had roughly 313,000 spam posts and around 700 accounts in the space of a month. I am also intrigued as to why you want a spammed vBulletin forum. Just out of curiosity were you looking for Spam bots or forums occupied by 13 year olds?
I hear tell this one gets tons of spam: http://forums.digitalpoint.com Maybe you can make the admin an offer.
To use it? I've tested new plug ins and new things, that's how I know whether its going to work for me or not, haven't you?
kmoneil - read the posts - jasmine was offering up her phpbb forum as a test forum which would be a fat load of use for a vB plugin.