You can use the vb
impex script to import photopost users http://www.photopost.com/forum/1156889-post2.html
This might help you although a little old.
Essentially what you want to do is get the users into vbulletin.
Clear your usergroups table.
Rerun install to integrate with vbulletin.
Then to make sure photos are owned properly maybe run this http://www.photopost.com/forum/1107165-post1.html