That's odd, and unfortunately I have no idea why this is happening on your site. I doubt it has anything to do with the version of vB you are running. I also doubt that the add-on you mentioned is removing any code from your core files.
I highly recommend setting up a local server/dev site on your hard drive for making such tests. However you choose to run a test for add-on conflicts, try adding the line:
PHP Code:
define('DISABLE_HOOKS', true);
to the top of your "config.php" file and see if the issue persists.