When installing with vbulletin on install you must specify all the proper info. If you get a login loop then the most likely culprit is you did not enter your cookie prefix on install. This is very important because in vbulletin 3 the cookie prefix and license number are encoded into the password cookie for login purposes. If you do not specify things correctly your not gonna be logged in right. Check your config-inc.php to ensure you have input the proper
vb license number not customer number
For vbulletin these cookie settings usually work and photopost must be set the same
cookie path /
cookie prefix bb
cookie domain blank or .domain.com
You can use phpmyadmin to change these in the settings table settings 171-173 or rerun install.php and specify the right settings.