Hi, I finally managed to get it working. The 3rd party integration question, asking where the "Users database" wasn't clear to me so I did not specify the vBulletin database but left the default "photopost" database.
So, whenever I would login to the photopost admin panel, it was logging me out out of vBulletin and vice versa. I also re-confirmed all the cookie settings.
Thanks for your time!