[kh99]

I can see two possibilities - $vilxh_visitor['musername'] is already set when the function is called, in which case the hook code is never called, or else your hook code is called but for some reason doesn't change $user['musername']. You can narrow it down by temporarily sticking something like die("fetch_musername"); in your plugin code and see if it stops there. (Shouldn't affect the adminCP so you should be able to get back in to take it out of your plugin, but if I'm wrong about that, put define('DISABLE_HOOKS', true); in your config.php).