I realise I'm digging up a very old thread here but I have this exact problem.
I run my forum (3.8.1) and use the vbulletin authentication for my site.
The majority of the site usage would be pages that are not part of the forum but are authenticated by vbulletin.
My forum is on a domain like this
w ww.mydomain.com/forum and my other pages are on a subdomain like this
pages.mydomain.com.
I have my session timeout set to 3600 at the moment because browsing through the non-forum pages of my site does not update the session tables.
I have path to save cookies set to /
I have cookie domain set to .mydomain.com
On all my site pages I chdir to forum directory and include global.php but still my sessions table doesn't get updated.
I have taken the extreme measure of including a hidden iframe that loads the forum index inside just to keep the session updated !
I'd appreciate any advice on this please.
Thanks.