if you create a virtual host on a specific port listening to any address, the creation doesnt make a 'listen' directive in
/etc/apache2/ports.conf
at least warn the user or allow editing of the /etc/apache2/ports.conf from the virtual server admin page.
Webmin should add a Listen directive already, although it won't necessarily be in ports.conf - instead, it may be added after the Listen in some other file, like httpd.conf or apache2.conf