It seems that the if I go to my apache httpd-server with only the ip-address it will always default to the first vhost-config domain which is alphabetically first configured in the sites-enabled folder.
Now I have the case that some strange domain has as a DNS-resolution to our server which is obvisously wrong but results in error entries in my error.log file. If I enter this domain name in my browser I end up on one of my sites, actually the first on configured in the sites-enabled folder as vhost-config. This is the same behaviour as if I enter the ip-address of the server in my browser.
To solve this problem I entered a new virtual-host config file with the name aaa-catherror.conf which handles this cases as described in http://www.virtualmin.com/node/15688
But I wonder if this is the best solution.