I was having this problem a few weeks ago with sites not launching in ISPConfig 3.
The reason for this apache behaviour has been explained to you in several posts already. Apache shows a vhost when the domain and IP match. If IP or domain is wrong, apache routes the request to the default vhost (/var/www).
Check the router configuration, the external IP of domain.com and www.domain.com
must be routed to the internal IP 192.168.11.7. If it is routed to any of the other internal IP's on that server, apache can not route the requests to this vhost.