[Paul M]

You can force a style on a particular usergroup (or groups) like this ;

Create a plugin and link it to the style_fetch hook.

Put this as the contents ;

PHP Code:

if (is_member_of($vbulletin->userinfo,5,6,7)) // Usergroups //
{
    $userselect = 0;
    $styleid = 11; // Style id //
    $vbulletin->userinfo['styleid'] = $styleid;
    $vbulletin->options['styleid'] = $styleid;
} 


Edit the usergroups and styleid to suit.