[Dave]

Edit: just noticed it's for vBulletin 5... nvm.

I originally thought this was for vBulletin 4, so this will not work for your forum, but I decided to just post the script here anyway:

You could use something like this in PHP to mass rewrite IMG tags that look like: [img width=123 height=123] to [img]. Create a PHP file in the root of your forum and then visit it in the browser or execute it through the command line interface.

I recommend executing it against a test database first since I didn't test it.
Also if you have a very big database, it may timeout.

PHP Code:

<?php

require("./global.php");
set_time_limit(360);

$q = $vbulletin->db->query_read("SELECT postid, pagetext FROM " . TABLE_PREFIX ."post WHERE pagetext like '%[img %]'");

while($row = $vbulletin->db->fetch_assoc($q)){
    $vbulletin->db->query_write("
        UPDATE " . TABLE_PREFIX . "post
        SET pagetext = '" . $vbulletin->db->escape_string(preg_replace("/\[img\s(width=(...)\sheight=(...))\]/im", "[img]", $row['pagetext'])) . "'
        WHERE postid = " . $row['postid']);
}