So you are not getting an error message in our program?
Basically cookies need to be set the same this should help Cookies and Login Problems!!
Both products should be set the same and as far as cookie prefix if your using vbulletin by default thats going to be bb. You also need the vb
license number entered in the photopost config settings.
If users get logged out between applications then your cookies are not set the same.
You can issue a query like this in phpmyadmin to update the cookie prefix
UPDATE cp_settings SET setting='bb' WHERE id=182