3.8.2 already has the reCaptcha option standard. Read your vbulletin manual for details.
OKAY - well I just switched from standard image verification to reCAPTCHA. Let's see if that makes a difference.
I've been getting a moderate amount of SPAM - maybe one post a day or so, and we ban the user, ban the IP address, and report the details to
http://www.stopforumspam.com/add
to help others.
Another thing I just modified is that I made it so that even usergroups "Users Awaiting Email Confirmation" and "(COPPA) Users Awaiting Moderation" must "Require Human Verification on Configured Actions," not just "Unregistered / Not Logged In." Maybe this will help.