I just googled "Delete account vbulletin" and the short answer was "No. Go die in a fire." or something similar.

The reasoning is it creates a mess... what happens to all the users posts? Do they get changed to a generic nick name "guest"? Then nobody knows who ever said it? Does it go to inactive account with no link? What happens if someone else then registers as "Dak"? Do they get all of your posts and karma now?

There are a few vbulletin addons that allow this feature.. but I can't find any here that look like they've been enabled. Probably with good reason.

ETA: Damn... that many comments in the time it takes to google. Get a life people