I run a couple of vbulletin-based forums too, so I'm well aware of the issue.
The thing is, spammers target these forums and they have scripts that will register new users. They then use those user accounts to post spam messages. Its an ongoing battle to deter the spammers. First it was email verification where you have to click the link in your inbox. Then it was these things:
Those are there to try to thwart scripts from creating accounts. But they still get better and better.
There are other ways to thwart them too. My favorite (and easiest) is to add another custom field to the registration page. The bot expects certain questions in a certain order, so it fills in the blanks accordingly. Change the order, or insert another question in the middle and it'll fail. Like how you have to enter your password twice in a row? Put a question right before that, so it'll bump their two passwords down and the blanks won't match.
I think this board tried to do the same thing with the whole "enter the capital letters in VerSyS" or whatever. Thats the built-in vbulletin verification, and you can enter several versions of the question. Say you only have 3 versions of that question that it rotates between. The spammer just answers the question once and runs the script. Its now got a 1-in-3 chance of getting it right each time.
Gotta vary the question, so its not the same every time, or vary the location, so its not the same order of questions every time.
Even still, they can get through. A lot of spamming bots come from IP addresses in china. There are blacklists out there that you can plug into vbulletin, and the system won't allow user account registration from those IP's. Many times they register to tons and tons of forums at the same time with the same username or email address. You can get blacklists of those too, and don't allow users to sign up with them. Or even just blacklisting email addresses @whatever.com that are known to be fake/spam domains.
The bigger your forum gets, the more it gets targeted by spammers. It helps having moderators who are regulars on the site too, that way it doesn't take too long for the crap to get deleted if it does slip past the defenses.