paidsub_build Plugin - HELP!!! :(

[Mko]

Hello everyone,
I've been trying to construct a plugin where, upon a user purchasing any sort of subscription on the Forums, it'd update their usergroup on my server.

Here's the Plugin information:
Location: paidsub_build
Code:

$uid_ps = $vbulletin->userinfo['userid']; 
$userid_ps = fetch_userinfo($uid_ps); 

$query = "UPDATE characters SET mgroup = 11 WHERE id = '{$userid_ps}'"; 

print_log("Applying Premium for: User ID - {$vbulletin->userinfo['userid']}, Changing group to - 11"); 

$db->query_write($query);

It doesn't seem to work sadly. Does anyone have any idea of how to fix it?

Thanks a bunch