There's an option enabled in the backend of vBulletin that limits any single member's signature to only appear once per page per thread. This is to help minimize any repetitiveness if a few members are repeatedly commenting back and forth. However, it is only limited to each given page of a thread. If a conversation carries on to a new page, the signature will again appear in each user's first post of that subsequent page.

Hopefully that makes sense, but if not, please let me know.