If your using subscriptions/memberships i'd use paypal (if your a verified member then you can process credit cards) this will work well with your vbulletin subscription manager to alter a members usergroup automatically after payment, which would then be reflected within photopost where of course you would have categories only viewable by those who have paid (i.e - using a new global usergroup named 'paidmembers' or whatever)
Or using CCbill or IBill have the entire photopost installation behind the ".htaccess" system (which would be setup when you sign up with CCbill or IBill) just have some ready made preview pix on your free pages, when a member signs up via credit card they get their username and password from the credit card processor, then just point them to the members gallery, bingo...there you'll have photopost, open & ready to show your content without the need for usergroups or anything like that...just globally protected with the ".htaccess" files supplied by CCBill & IBill...
I use Paypal...not very hackable you see
- of course both ways are good.
-T