well if you move Photopost to the new server with
vb your still going to need to import the users into vbulletin database maybe you could use vbulletins impex importer for this as I think they have a script to import photopost users
Now you could then follow the advice in various threads on switching integration or hire Photopost to do an install which would switch things
Then this script could be run to realign the userid's to the new ones for the users you imported
Reassign userid's when switching integrations to VBulletin
There is multiples steps to do what you want to do.