Personally, I believe CAPTCHA is the way to go. Its simple, unobtrusive, and allows a real person to post immediately.
This method requires a new user to type in a series of letters and numbers that are displayed. The displayed letters and numbers are randomly generated and are displayed as a photograph. This makes it virtually impossible for the automated register's to read the letters and numbers.
This can be done via Admin CP > vBulletin Option > User Registration Options; set Image Verification to yes.