I've had a think about this and using the vBulletin session table is probably a better way to do this. If this is used the total number of online users can then be accessed across the whole site (links and forums).

Ideally I would like to be able to include the URL the user is browsing on the Links side in the session table. It may then be possible to modify the 'Who's Online' page in vBulletin to use standard phrases for what users are looking at in the the Links part of the site.

There are quite a few include files in vBulletin that handle the sessions so hopefully it is a case of adapting the relevant code and adding it to the Auth_Bulletin plugin.