Fredrik, that is the exact wordpress plugin i used, this helps you bridge users and posts if you wish..
I then created a widget using an 'external login php script for non vbulletin pages' which allowed me to have the welcome box on the wordpress page.
then used the following script (create a plugin in vb) to call the wordpress header and footer: (note, this is specific to my theme, you will need correct paths to your own theme)
define('IN_VBULLETIN', true);
include '../wp-load.php';
// Nav bar
ob_start();
include '../wp-content/themes/arras-theme/includes/nav.php';
$wp_nav = ob_get_clean();
// Footer message
ob_start();
include '../wp-content/themes/arras-theme/includes/footer-message.php';
$wp_footer_message = ob_get_clean();`
use the vars to include it into vb header and footer $wp_nav $wp_footer_message
this will generate errors bcoz of a conflicting sanitize_url() .. this needs to be changed in wordpress to something like wp_sanitize_url()