Configure apt to use proxy in vagrant box

I'm using Vagrant to setup a virtual machine for testing. I would like apt inside the virtual machine to use a proxy on the host machine to allow caching of all the downloads to persist between instances of the virtual machine running, not only for improved speed but to allow me to start the vagrant instance when I have no internet connection.

I've setup the proxy which is running, and I thought I'd told apt to use it by setting this in the Vagrant script:

config.vm.provision :shell, :inline => 'echo \'Acquire { Retries "0"; HTTP { Proxy "http://10.0.2.2:3128"; }; };\' >> /etc/apt/apt.conf'
config.vm.provision :shell, :inline => 'echo \'Acquire { Retries "0"; FTP { Proxy "ftp://10.0.2.2:3128"; }; };\' >> /etc/apt/apt.conf'
config.vm.provision :shell, :inline => 'echo \'Acquire { Retries "0"; HTTPS { Proxy "https://10.0.2.2:3128"; }; };\' >> /etc/apt/apt.conf'

And it's working partially i.e. the initial requests for sources hit the cache and work when my wifi connection is disabled but Squid is running and has some entries in it's cache. I can also see some requests hitting Squid from Squids log file:

1367492453.816     34 127.0.0.1 TCP_REFRESH_MODIFIED/200 592 GET http://security.ubuntu.com/ubuntu/dists/precise-security/Release.gpg - DIRECT/91.189.92.181 -
1367492453.987    168 127.0.0.1 TCP_REFRESH_MODIFIED/200 49973 GET http://security.ubuntu.com/ubuntu/dists/precise-security/Release - DIRECT/91.189.92.181 -
1367492453.999    325 127.0.0.1 TCP_MISS/404 588 GET http://us.archive.ubuntu.com/ubuntu/dists/precise/InRelease - DIRECT/91.189.91.13 text/html
1367492454.113    114 127.0.0.1 TCP_MISS/404 596 GET http://us.archive.ubuntu.com/ubuntu/dists/precise-updates/InRelease - DIRECT/91.189.91.13 text/html

However, the bulk of the download of packages are not being cached and doesn't appear to go through Squid at all i.e. I can see large network usage when they're downloaded and they don't appear in the Squid access log.

So my question how do I configure Apt to use a proxy for all it's requests?

btw I also tried configuring by setting the environment variable http_proxy:

 config.vm.provision :shell, :inline => "echo 'export http_proxy=http://10.0.2.2:3128' >> /etc/profile.d/proxy.sh"

This has the same effect – some requests appear to be hitting Squid but not all of them, and particularly not the actual packages.