what login you referring to? Your forum login box?
Nothing on photopost install changes your forum settings so lets suggest this.
Your vbulletin forum should be set as this
cookie prefix bb ( this is set in your config.php file )
cookie path /
cookie domain .domain.com
photopost should be set to this
cookie path /
cookie prefix bb
cookie domain .domain.com
vb license number must be correct in the Photopost config-inc.php file and you can find the license number at the top of the
vb config.php file.