It sounds like you have a wildcard DNS entry in place that directs any requests for *.main.com to the same IP address. In this case, Apache will route the request to the VirtualHost that matches. If no VirtualHost is an exact match, it will use the first one defined. So you’ll want to ensure that main.com is defined first:

<VirtualHost *:80>
    ServerName main.com
    DocumentRoot "/www/main.com"
</VirtualHost>

<VirtualHost *:80>
    ServerName 1.main.com
    DocumentRoot "/www/1.main.com"
</VirtualHost>

If you have the VirtualHosts in separate files, the order they are loaded will determine it. So use a filename like: 000-main.com.com to ensure it is loaded first.

For more info, see the Apache documentation and check out this tutorial:

• How To Set Up Apache Virtual Hosts on Ubuntu 16.04