well photopost 4.0 comes with a intergration hack when you install it, you just choose intergrated hack, and vbulletin 2.20+ in your config for photopost you just setup so that it has your vbulletin database for the forum, and your photopost database for your gallery, and your good to go..
example with my site:
www.pixelfx.ca/index.php
I use vbhome, vbulletin, and photopost 331, soon to upgrate with photopost 4.0 for that site...