Well, the topic really says the problem. In vBulletin I know if you don't create your links a certain way, the session will be lost. It's been a really long time since I've worked with it so sorry I can't give any specifics on how to fix. It's very simple though, as far as I can remember, so probably looking at any other link would instantly show. I believe there's built-in functions for creating links so that sessions don't get lost.