Hi,
I have created a specific function for bridging Icy Phoenix with any other software and being able to keep track of user data changed (such as email and password).
The function is in class_users.php:
    /*
    * User profile update: this function is called when username, email and password are changed in a user profile
    */
    function profile_update($target_profile_data)
    {
        global $db, $config, $user, $lang;
        /*
        $target_profile_data = array(
            'user_id' => '',
            'username' => '',
            'password' => '',
            'email' => ''
        );
        */
        //print_r($target_profile_data);
        return true;
    }
The work is still incomplete, because I should have coded a hook where customized files could be hooked, but didn't do that so far... anyway the function is still a great helper, since it is connected to any crucial data change in a user profile.
Regarding your issue, try to print the var just to make sure that the var has been properly "globalized".
Try to add this somewhere 
print_r($cms_config_vars);
Alternatively pass the DB throug a $_POST var.