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: