[kh99]

$db->query_first() will return an array (even if you are selecting only one field), or else NULL if it fails. So you should check for NULL, but maybe just try using $a['password'] and see if that works.

Edit: also I don't think $vbulletin->userinfo has been updated at that point (since the user was just created), so try using $userinfo and/or $userid instead. And of course using those is probably better than doing your own query.