It is unfortunate that so many people compare Vagrant and Docker. While there is overlap, Docker is mostly not viable as a dev environment tool alone, so it isn't a fair comparison. The main reason is because you have to be using Linux (and a recent Linux) as your main dev system, and in practice this is very rare. Move beyond indie developers and for all intents and purposes Linux desktops are non-existent (Vagrant is in use by companies like BBC, Expedia, Intuit, etc. and I can tell you most devs don't know how to use Linux let alone run it as their primary dev platform).
BUT, I agree that putting your dev environment in a Docker container is absolutely _amazing_, and there is a KILLER Vagrant/Docker combo.
The killer combo is actually running Vagrant to spin up Docker-ready VMs, then using Docker inside that to develop. This lets people use Docker on Windows, Mac, and Linux. You get the fast iteration time because all your state is actually in a container, so you just docker kill and run as usual.
In fact, an upcoming version of Vagrant is adding Docker as a provisioner, so you can `docker pull` containers down as part of `vagrant up`.
And I published Packer[1] templates to build Docker-ready VirtualBox/VMware/AWS images that are Vagrant-ready: https://github.com/mitchellh/packer-ubuntu-12.04-docker
[1]: www.packer.io