Hello
What is your server path to vbulletin? I would tend to think Photopost being a white page while alot of server's tell you the error yours is not. You entered an incorrect path to the forums so we can not find global.php You can edit config-int.php and look at the $vbpath variable and correct the vbulletin server path.
Cookie settings also you most likely forgot here to enter bb as your cookie prefix. You can rerun install or using phpmyadmin make setting 172 in the photopost settings table bb
Your vbulletin cookies should be this
cookie path /
cookie domain .domain.com note the dot in front.
cookie prefix by default is in the config file of
vb and is bb
photopost should be set as this
cookie path /
cookie domain .domain.com
cookie prefix bb