User would need to specify:
Vagrant VMs appear to have a host-only adapter at eth1 which we could then proxy to. However, it seems the cleanest way to get to the IP of that interface is to parse the output of vagrant ssh -c "ifconfig eth1", which is not very clean at all.
vagrant ssh -c "ifconfig eth1"
Vagrant must know what this IP is internally, though. Maybe we could write a Vagrant plugin?
Probably related to #3.