Running Two Services on Port 80 – How to Configure

This is called Reverse Proxy, you can redirect all trafic on port 80 to one Apache server, configure 3 vhost (1 per subdomain) and in the 2 vhost handling non local website use the Apache ProxyPass directive to tell apache to load content from another server.

nginx can also act as a reverse proxy. You can forward all request to nginx then nginx will load content on one the right server depending of the subdomain.

I would use the Apache proxypass feature, it's easier to setup from my point of view.
You can also start a 4th apache server that will only act as reverse proxy. In this case all of the 3 vhost will use proxypass to get content from the right server.

See http://www.raskas.be/blog/2006/04/21/reverse-proxy-of-virtual-hosts-with-apache-2/ for an example, you case is the same execpt that you have a third subdomain (so you need 1 more vhost).

Basically when you put in "domain.com" the web browser actually goes to "domain.com:80" (port 80 is the default HTTP port).

You can only assign one application per port on a computer, so only one can listen to the default HTTP port (80) and the other will have to use another port.

There are three ways around this:

1) the proper way would be to use nginx as your main web server (i.e. listening on default http port 80), set Apache to listen on another port (e.g. 8080) then set up a 'reverse proxy' (google is your friend here). This is the ideal set up. It takes a bit more effort to configure but there are hundreds of guides out there that are really helpful.

2) use nginx to serve the django websites (if that is possible?)

3) a quick and easy way (although not recommended), assuming the PHP sites are for webmasters only and the django sites won't need https, you could set up the PHP sites to run over HTTPS. That way you could have Apache running on port 80 (default HTTP) and nginx running on port 443 (default HTTPS). Just put 'https://' instead of 'http://' and you'll get to port 443.