Forgot this thread was left dangling. Vagrant is just a dev tool (a VM manager) that doesn’t strictly need a config file to be useful, and certainly not a dedicated image (it’ll quickly bring up a VM capable of hosting Docker & running Discourse scrips like discourse-setup & launcher just fine).
There’s already a Vagrantfile at discourse_docker/blob/master/Vagrantfile
and simple instructions at the end of discourse_docker/blob/master/README.md for hacking on discourse_docker.