There should be something in the admin panel. If not, maybe a mod to be installed? (I am not using VB, but I know there's a mod for phpBB that does this. So I think the VB developers have thought about this too

In the vB admin panel, click vBulletin Options in the left frame. Under the category entitled User Registration Options, click to Require Unique Email Addresses and also click to Verify Email address in Registration. If it's a problem that bots are auto-registering on your forum, youmay wish to also enable Image Verification.

I usually just throw a couple bucks to experienced Sitepoint members who know how to fix complex problems on my site. They are usually willing to do quick complex tasks for a couple bucks.

The answer is here

Well there ya go