Error Handling & Logging 
Path to Error Handling and Logging options: Settings > Options > Error Handling and Logging


Log Database Errors to a File
To log all database errors to a file, enter the path to the file. The file will be saved as {filename}.log.
Note:
The directory in which this file is to be created must be writeable (chmod 777) by the web server.
Log Failed Admin Control Panel Logins to a File
To log all failed Admin Control Panel login attempts to a file, enter the path to the file. The file will be saved as {filename}.log.
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Note:
The directory in which this file is to be created must be writeable (chmod 777) by the web server.
Log PHP Errors to a File
To log all PHP fatal errors to a file, enter the path to the file. The file will be saved as {filename}.log.
Note:
The directory in which this file is to be created must be writeable (chmod 777) by the web server.
Log Emails to a File
To log all emails to a file, enter the path to the file here. The file will be saved as {filename}.log.

You should enable email logging only if you suspect problems with the email system within vBulletin.
Note:
The directory in which this file is to be created must be writeable (chmod 777) by the web server.
Maximum File Size of Error Logs (default: 1048576)
Note:
1048576 bytes = 1 megabyte
If you want your vBulletin error logs to be rotated when they reach a certain size, enter the maximum file size in bytes.

When a log file reaches this size, it will be renamed as {filename}{unix timestamp}.log and a new file will be created.

Set this value to 0 to disable log rotation.


Disable Database Error Email Sending (yes/no)

To prevent vBulletin from sending email to the $config['Database']['technicalemail'] address you specified in config.php, set this value to Yes.

Error reports about database connection errors will still be sent.

We do not recommend that you set this value to Yes unless you are logging database errors to a file.
User Contributed Notes: Error Handling & Logging