Might be wrong about this but here we go.
In vbulletin go to your your admincp> settings> Cookies and HTTP Header Options
and check the options in there. They should be as follows:
Path to Save Cookies set to /
Cookie Domain set to .yourdomain.com
Obviously the "your domain" bit should be replaced with your actual domain name. Don't forget that this should start with a . (dot)
Now in photopost pro
Admin> all settings> Cookie Settings
Now check that the cookies setting actually match with vb
the setting should be:
PhotoPost Cookie Path /
Forum Cookie Prefix bb
PhotoPost Cookie Domain .yourdomain.com
also just beneath this is the forum url make sure that data is correct.
Reckon that should be the problem.