Apache2 + Disable Default Virtual Server

You need to keep port 80 open as all your content for the domain.com/www vhost travels through 80 as well.

There is a default vhost setup by apache, you just need to comment it out in your httpd.conf.

To edit the config in webmin:

 Webmin -> Servers -> Apache Website -> Global Configuration

Give it a slow read through and you'll find the default vhost code block, just comment it out. This is worth it as you may want to tune settings later anyways.

Andre(OP) determined the default vhost definition was in /sites-enabled/default-000

Here is the section you are looking for:

### Section 2: 'Main' server configuration
#
# The directives in this section set up the values used by the 'main'
# server, which responds to any requests that aren't handled by a
# <VirtualHost> definition.  These values also provide defaults for
# any <VirtualHost> containers you may define later in the file.
#
# All of these directives may appear inside <VirtualHost> containers,
# in which case these default settings will be overridden for the
# virtual host being defined.
#

Comment out these Directives within Section 2 using a hash:

#ServerName
#<Directory />
#<Directory>
#---comment out all contents of this directory block---
#<Directory /var/www/html>
#---comment out all contents of this directory block---
#</Directory>
#DocumentRoot