Debug Mode 
When developing for vBulletin or troubleshooting issues, you often have to enter Debug Mode. Debug Mode provides you with more information about how vBulletin runs some examples include: page generation times, templates used on a page, the memory used on a page and how many queries it took to generate the page. Debug Mode should not be used on a live production site unless absolutely necessary. It can expose information that could weaken security on your site.
Note:
The config file is located in your includes directory.
Entering Debug Mode
To do this, you need to edit your config.php file and add the following line:
$config['Misc']['debug'] = true
This line of code can be anywhere in the file after the <?php and outside of a comment. Once done, save the file and overwrite the copy on your server.

Turning off Debug Mode
Edit your config.php file and remove the above line of code or comment it out by placing two slashes ('//') in front of it. Save the file and overwrite the copy stored on your server.
User Contributed Notes: Debug Mode Add a Comment