1. You would need to upgrade your photopost to the latest.
2. You would need to install vbulletin
3. You would need to use
vb impex importer to import your photopost users.
4. You would need to reinstall PhotoPost to integrate with vbulletin.
There are threads here on switch integration as a search term that go into greater detail.
We also offer this service and if you would like to know how much it would be you can inquire through the contact us link in the header. I can tell you
vb charges you 160.00 to install there forum if you get them to do it whereas I can tell you our charge to reinstall photopost and install a forum is less than that.
Of course if you do the steps needed when switching an integration you can save yourself any install fees.