[FullyTested]

You could insert this in the global.php file where the following hook is:
($hook = vBulletinHook::fetch_hook('style_fetch')) ? eval($hook) : false;

PHP Code:

if ($vbulletin->userinfo['usergroupid'] == 10)
{
    $vbulletin->userinfo['styleid'] = 2;
    $styleid = 2;
}
else if ($vbulletin->userinfo['usergroupid'] == 20)
{
    $vbulletin->userinfo['styleid'] = 3;
    $styleid = 3;
} 


replace the above numbers with those of your usergroups and desired styles