vBulletin Manual - Control Panel Log

The Control Panel Log is where all actions performed within the Admin Control Panel and the Moderator Control Panel can be viewed or deleted.

Control Panel Log Viewer

Log Entries to Show Per Page - Number of entries to show on each page.
Show Only Entries Relating to Script - Filter the logs viewed by the script that generated the log entry, this will still include files that may have been deleted after the logging.
Show Only Entries Generated By - Filter the logs viewed by the user that generated the entry.
Order By - The logs can be sorted based on Date, username or action performed.

Prune Control Panel Log
Pruning of the Control Panel Log should be performed to reduce the size of the database, alot of space can be consumed by this data.

Remove Entries Relating to Script - The script to remove log entries for.
Remove Entries Logged by User - The user to remove log entries for.
Remove Entries Older Than (days) - Removes logs older than the length specified in days.

Note:

To access these parts of the Admin Control Panel you may need to edit your config.php please refer to Editing the vBulletin Configuration File for further instructions.