It was the index.php file, not the forums (wasn't sure, it's been a while since I had to look)
but this didn't work?
Code:
/**
* If you want to move this file to the root of your website, change this
* line to your vBulletin directory and uncomment it (delete the //).
*
* For example, if vBulletin is installed in '/forum' the line should
* state:
*
* define('VB_RELATIVE_PATH', 'forum');
*
* Note: You may need to change the cookie path of your vBulletin
* installation to enable your users to log in at the root of your website.
* If you move this file to the root of your website then you should ensure
* the cookie path is set to '/'.
*
* See 'Admin Control Panel
* ->Cookies and HTTP Header Options
* ->Path to Save Cookies
*/
//define('VB_RELATIVE_PATH', 'forums');