I know of forums that use a similar registration requirement and it does stop a lot of them. They can be pretty annoying reading the letters sometimes for real members. I think my favorite ML forum has had very few bot posts or even bot registrations since the site admin implemented it. Ive only seen 2 in several months get far enough to make a post.

We dont use vBulletin though, we use a open source software and have no advertisers to keep happy.