I have a slightly different problem here and on the other vBulletin board I frequent. It seems to log posts as "read" (and auto-logout if you haven't clicked "remember me") after a default period of time. The SMF forum did not do this. So based on that n=2, I'm going to say it's a problem with vBulletin. It annoys me because I want to come back and read posts that are new, but they are automatically marked as read--which makes the "new posts" button not very useful.