When you edit the first post of a topic in vBulletin, it doesn’t update the topic title, even if you update counters in the Maintenance section.
One of the first posts I found on the subject was Bug with editing title. The info in this thread is wrong for the latest version of vBulletin 3. A post on a different forum, Editing thread title, has it correct.
The key is that the post titles and topic titles are separate, even though topic starters enter them at the same time (thus making them almost always the same). So here’s what you actually have to do to change a thread title in vBulletin: