Please clarify your configuration. Are you suggesting that you have a main URL that is set up to be redirected to vhosts? Is this a load balancing scheme, round robin,... Are these vhosts on the same system?
The request is processed by a virtual host with a matching ServerName or ServerAlias . If no one matches the url, the first virtual host defined will process the request.
You can dump some configuration settings, including all vhosts, with httpd -S or apachectl -S .
You also might consider defining a custom log format including the %v specifier to print out the ServerName handling this particular request.