One item though, Bots from complex programs like SENUKE (this things crazy) are aware of the post limit link measure and can be set to keep garbage posting until a link is allowed.
I can't speak from experience for this particular solution but I assume this means some bots will end up making 11 garbage posts before leaving to ensure they get their link on the site, but it should cut down on simple bots for sure.
--> Just food for thought.
I totally get not taking advice from a JR. with >50 posts, but read up on Q&A verification as I believe it's your best bet, as it poses a totally unique problem that bots can't logically figure out.
Simple to do too.
Admin CP -> vBulletin Options -> Human Verification Manager -> Question and Answer Verification
"While balancing on a piece of wood, two inches by four inches known as a 2x4, John and his friend Sally both spotted a dalmatian in the passenger seat of a truck with sirens that was headed to put out a fire. Guess the color of the vehicle that the dalmatian was drooling in."
Anyways, un-subbed now, best of luck.