Your missing one of two things for a successful login
First off your
vb cookie prefix is stored in the
vb config file and by default is bb.
Second I would suggest that you set the cookie domain in both applications to .domain.com so users are logged in whether they visit your site at
http://domain.com or
http://www.domain.com
Third make sure the cookie path is / in both applications
Fourth in your Photopost config.inc.php file you must place the proper
VB license number not customer license in that file where it states
$vblicense="12345678";