Wireshark installed on both host and guest (this is an easy way to get dumpcap installed on the guest even if it's headless).
To get an insight into the traffic going around the place, I've been using Wireshark and it's ability to capture remotely, it's really simple so I thought I'd write down my "recipe" on how to do this in case it's useful. you get the picture :) This is great but having everything on the VM can make it a bit trickier to debug what's going on - and with a website that talks to an API that talks to MySQL, that all lives on a VM with port forwards, you can see the problem :) The joind.in project uses a really nice vagrant setup for its dev platform - which is much needed these days as we move away from a single LAMP stack install to a website plus another website, which talks to an API and caches in redis and deploys with.