The reason vBulletin stores usernames into posts is because it supports the option to let guests post without registering an account. When permissions are set properly guests can reply/start threads and type in a username to be shown.
So like, when an account is deleted, all of the posts from that account are marked as guest and have no information associated with them, but it still shows their username.
That totally makes sense now. So their guest feature was essentially a downfall of the entire system. I wonder why they never saw this as a potential issue down the road.