You can do two things:
1) run phpMyAdmin from a subdirectory of mydomain.com. In that case you have to put the phpmyadmin directory in /var/www/mydomain.com. You don't have to create another virtual host, because phpMyAdmin is inside the first virtual host.
2) run phpMyAdmin in its own virtual host, for example: http://phpmyadmin.mydomain.com/. When I look at your configuration file, I think this is what you want. You have to use
VirtualHost {
Hostname = phpmyadmin.mydomain.com
...
}
You must of course make sure that the hostname resolves to the right IP address.