how to keep a user's vBulletin session alive (Where is it in the source code?)

[Psalmist]

How do vBulletin sessions work? It appears that it doesn't use PHP sessions. The source code is so complex, I can't find it.

I want to make a simple php page that serves no purpose except to refresh the user's vBulletin session.

Can someone show me the part of the vBulletin source code that does this (so I can simply copy it)?

(We're putting a chat app on our forum site. As long as users are chatting in the chat room, we want them to still be "logged in" to vBulletin. I can make the chat app periodically invisibly retrieve the php page that will refresh their vBulletin session.)

Can someone help?

Thanks!