I have significant experience with vB and run a large board.
Spammers have an automated program which you can find if you look. It automatically registers and posts on vB and similar boards. You will find posts for porn sites and cellphones, ipods etc. Most of them are Western Union scams.
One thing I noticed when I signed up this morning was that no email confirmation was required. That is the first step and should be 'on'.
And if the admins are reading, setting 'GD' in Image Processing Library
and 'GD True Type' in the Image Verification Library
(both under Image Settings in the CP) will cut down on 99% of automated spambots because they can't get past the harder CAPTCHA library.
Just doing my bit to help