Second attempt at this post got logged out in the middle of first attempt. The one cookie I noticed vanish was session_lock. I've got the following bb cookies by the way bbcursession, bblastactivity, bblastvisit, bbpassword, bbsessionhash, bbthread_lastview and bbuserid. I've got screenshots of all the cookies both before and after if you want them. Unfortunately without values, but with domain, path, name, expiration, and site domain.
Not sure if anyone has mentioned this anywhere else, but I will put it here. If I turn off the computer and come back to the forums the next day, the forums have logged me out even though I have "remember me" checked. When I go to reply to a post, it prompts me to log in and then tells me my information is incorrect. so then I have to go to the top of the page and click log in for it to automatically log me in.
I know what's causing the seemingly random logouts.
I'll spare you the vBulletin-technical details, but I have applied a band-aid that will (or should I say; should) result in you not being logged out for 24 hours of inactivity (as in, not browsing the forum for 1 day).
I am working on a more permanent fix, but for now this is good.