vBulletin Manual - Merge Users

If you wish to merge two users’ accounts into one, perhaps because one user registered twice, go to Users > Merge Users.

Simply specify the source and destination users. Posts, threads, calendar events, and private messages of the source user will be changed to appear to have come from the destination user. The source user’s post count, reputation, buddies, and ignored users will be added to the destination user.

Permissions will not be changed and moderator status will not be transferred; these must be respecified.