Re: vBulletin Upgrade
No, option #1 is definitely what we used prior to the upgrade, as it was the only available option for tracking read threads prior to 3.5. The reason you may not have noticed it is because of this aspect: "Individual threads are marked as read within a session via cookies.". That's how vB marks individual threads read (within a session) before the user clicks "Mark All Forums Read" when he or she is finished. Under this system, all threads will be marked as read if a). the user clicks "Mark All Forums Read" or b). he or she does not do this and 15 minutes pass.
I think we're OK from a bandwidth standpoint that none of the options should cause any problems.