Well if your vbulletin login is not working then photopost wont work either
I would suspect this is a cookie issue.
I beleive
vb has a cookie login fix on there support forums but I can tell you how to fix Photopost at least and how to fix vbulletin on the backend
If you use phpmyadmin from your control panel you can set photopost cookies by viewing the settings table
Change settings 171-173 to the proper cookie settings. I recommend this
cookie path /
cookie prefix bb
cookie domain .domain.com
Now with vbulletin you can view there config table and find the cookie domain setting and clear this and login to
vb admin and set cookies to / and .domain.com
The cookie prefix for
vb is located in there config file and by default is bb