[Nibbles]

I figured it out, check the checkbox in the header of the table, then in the bottom select "Mark as read". This will force the "unread messages" number in the database to refresh.

Either a SQL DELETE was used to get rid of the messages, or there is a bug in vBulletin which doesn't issue an update after a mass delete (I don't think vBulletin has such a control panel that can delete PMs, last I checked a few years ago).

Anyways, this solution worked for me. That bold "1 Unread message" was driving me crazy too.