Please create the websites in ISPConfig. Ddo not use webmin together with ISPConfig and do not create any sites manually in /etc/apache2/sites-available!
You do not need to configure anything manually, the complete vhost configuration is done by ISPConfig. To preare your server for ISPConfig, please use one of the guides listed on the ISPconfig documentation page.
Is your server behind a roter? Then you must use your interna (privete) IP address for the websites and not your external IP.