Much simpler, better targeted solution:
IsBot - Registration time check
No anti-spam subscription keys necessary, no complicated install. 90% of spam out there is generated by botnet zombie computers. The entire point of spambots is speed, therefore they only spend an average of 3 seconds on the register.php page. This mod stops any registration that takes less than 15 seconds, never bothers a human.
This combined with any of vBulletin's built-in human verification measures makes your board Ft. Knox to spam. Since my install of this I have had zero spammers, out of more than 5,000 tries - and never interfering or inconveniencing a single human.
I would be happy to install and configure this for you.