I agree that the Vagrant/Docker combo is awesome. I see them as solving completely different problems. To me, Vagrant is for running complete systems while Docker is for running isolated processes (quite commonly within a system managed by vagrant).
Docker source itself ships with a Vagrantfile so that you can get an example development environment up and running should you be running on an old Linux kernel that doesn't sport Docker's current runtime requirements. Just 'git clone firstname.lastname@example.org:dotcloud/docker.git docker && cd docker && vagrant up'.