Editing the vBulletin Configuration Files 
As of vBulletin 5 there are two configuration files. There is the new vBulletin 5 presentation config.php file, and the core configuration file. We will first go over the legacy configuration file as it contains your database information. The legacy configuration file is always located in the upload/core/includes/ folder of your vBulletin package.
Before uploading the vBulletin files to your web server, you must edit both vBulletin configuration files to tell vBulletin how to communicate with your database and server.
To edit the configuration files, you will need to open the file in a text editor such as Windows® WordPad or another recommended editor.
Please use a recommended editor to avoid issues with the formatting and display of the file. PHP files are standard ASCII/ANSI plain text files and should not be saved as UTF-8 with BOM.
Format of the config.php Files
The vBulletin configuration files are in PHP. However if you follow these instructions, knowledge of PHP is not required to edit these files. The files are a series of variables. When setting a variable, you enter the value to the right of the equal sign. If the value is text rather than numbers it needs to be in single quotation marks.

When making changes to the file, the edit should always be to the right of the = sign between the single quotes. For example:

Default: $config['admincpdir'] = 'admincp';
Correct: $config['admincpdir'] = 'myadmin';
Incorrect: $config['myadmin'] = 'admincp';

Make sure that you edit the info to the right of the equal sign. Do not edit the names in the brackets to the left of the equal sign.

User Contributed Notes: Editing the vBulletin Configuration Files Add a Comment
Dominik Hahn 29th Sep 2005, 12:56am
You should mention that the Debug-Mode can be turned on with:

$config['Misc']['debug'] = true;
Add a Comment