non-VB using VB - password change

[DHDesign]

we have an intranet system we've built and it utilizes vBulletin's login system and so forth...everything works like a charm so far.

question is: there is a My Profile section that users will use to change their password...we dont want to use vBulletin's password change page, cause we have our own design.

(the admin initially sets up the user with a temporary password as the site does not allow new registrations or outside access...we know who everyone is)

will it work to simply update the vb_user table as such when they go to the My Profile section:

$newpass = $_POST['newpass'];
$currentpassword = mysql_query("SELECT passworddate, salt FROM vb_user WHERE userid = '$userid' LIMIT 1");
$current_pass = mysql_fetch_array($currentpassword);
$vbsalt = $current_pass['salt'];
$pass_hash = md5(md5($newpass) . $vbsalt);
$passdate = date('Y-m-d');
$updatepassword = mysql_query("UPDATE vb_user SET password = '$pass_hash', passworddate = '$passdate' WHERE userid = '$userid'");

i implemented this and it worked, BUT, now i have users trying to log in a couple of days later and now all of a sudden, they cant...so the admin needs to reset the password and they can get back in again.

so is something wrong with how we are changing the password?