Just being serious for a moment, to change a username isn't that difficult in Vbulletin, but it does depend on how many posts the user has made and how far back in the database they are.
There are two ways to do it, both of which require Admin time to sort out.
You can edit the username itself, or you can create a new username and merge the posts of the two users to the new name.
I've done it on our own forum, but it's a faff if you're busy with other stuff to get done.
Generally speaking I don't allow it unless there are good reasons for doing so.
Peter