You always have to setup the domain to work on your server first. I would assume both domain names would work there then you go into the forum and photopost admin areas and change your urls to the new domain once it propogates.
I have done de change of domain name but I have problem when I try to access to the vbulletin forum becouse get the error:
Warning: require() [function.require]: open_basedir restriction in effect. File(/var/www/vhosts/olddomain/httpdocs/photopost/config-int.php) is not within the allowed path(s): ([path]:/tmp) in [path]/forum.php(653) : eval()'d code on line 48
Warning: require(/var/www/vhosts/olddomain.com/httpdocs/photopost/config-int.php) [function.require]: failed to open stream: Operation not permitted in [path]/forum.php(653) : eval()'d code on line 48
Fatal error: require() [function.require]: Failed opening required '/var/www/vhosts/olddomain.com/httpdocs/photopost/config-int.php' (include_path='.:') in /var/www/vhosts/newdomain/httpdocs/forum.php(653) : eval()'d code on line 48
I have edited the config-int.php file and I get the same error.