Ok, here's how it works. When a user is new on the forum, and has no signature set, it will of course not be shown on posts. When the user sets a signature, all his new posts will show the signature, but they will not show on his old posts written prior to the signature set. That's how vBulletin handles signatures. I would prefer it to be shown on old posts as well, like in most other forum software, but like I said, it's not how it works in vBulletin, and to change that would require code changes from the vBulletin makers.