Monitor the bandwidth usage per VirtualHost in Apache
If you do not have any hosting control panel, you probably will not be able to measure the bandwidth usage per virtualhost. You will need additional Apache module to log input/output request and then you can use script to parse the logs and save the usage to your database.
One of the available Apache modules is mod_logio. This module provides the logging of input and output number of bytes received/sent per request. For more information on this module, you may refer the following: