I just changed settings on the board to better mark individual posts as read or not read.

Since this is a one-time change, every post older than 10 days just got marked as unread for you. But the advantage is, from now going forward, the board will record and display read/unread. This should make the New Posts and Recent Posts features more useful for you.

All this thanks to the superior support at vBulletin, helping us learn yet another capability in our massively full featured board software.