Adding this Code to WP-Config Without Breaking Everything

I have a vBulletin login box I’d like to display on my wordpress installation but it’s causing a lot of hassle. I eventually found out how to do it properly without the two systems functions interfering thanks to this post: https://www.ads-software.com/support/topic/vbulletin-login-in-sidebar?replies=7

All I have to do is add this code to my wp-config:

$root = $_SERVER['DOCUMENT_ROOT'];
$curdir = getcwd ();
chdir("$root/forums/");
require("$root/forums/global.php");
chdir ($curdir);

After doing it, the login box works perfectly but some random parts of the WP admin panel stop working. Plugin settings throws up a permissions warning and uploading images to posts asks me to try again. Once I remove the code it all works fine again but I really need it to be in the config file.

Any ideas?

Thanks.