Right, it seems that vBulletin does have a tend to log you out 'early' if you don't use the remember me option - I'm not completely sure about what leads to this behaviour, but I know it does, so I always use the remember me function. Likewise it seems to have problems sometimes with read/unread posts not being correctly recorded, I have to say that this seems intermittent for the most part though; I notice it now and then but not with every forum/thread.
If you persist without using the remember me option I'd be interested in some stats on when it's kicking you out? It might be a tad laborious, but if you noted a time you log-in and the time when it kicks you out, and an approximation of how long after your last 'link click' that was, it might help me track down what's happening.
If you do give 'remember me' another try, I'd be interested to know how persistent the read/unread marks issue is too...
Hopefully these issues might improve/vanish in the coming weeks though, I'm hoping to upgrade the vBulletin core we're using...
Regards,
Rob.