If you need delete all messages of a user in a vBulletin forumI leave you two different ways to do it. A graph and another attacking the database.
If the user has a normal amount of messages, the graphic form with vBulletin's own tool is the best and least dangerous.
It has happened to me many times that when moderating a forum we see that we have to delete all the messages of a user, either because they are not appropriate, or because they are spam or even because the user asks us to delete his profile and all his messages.
This tutorial is for vBulletin 4.xx versions I don't know if it works for 5.x because I haven't tested it nor do I know the structure of its database.