Docker vs. Vagrant

Docker and Vagrant are somewhat similar technologies, so what are their relative strengths and weaknesses and when should you choose one over the other? I'm fairly new to both, but here's a compare-and-contrast based on what I've learned so far.

The tl;dr is that Docker is really best for running applications in production and fast testing across linux flavors. Vagrant handles Windows and OS X in addition to Linux, and is good for automating building and packaging of software, and testing where you need a full OS stack running. If you need to build a .dmg or a .exe installer for your software, Vagrant is a good place to do that.

Feature	Docker	Vagrant
Guest OS	Linux only (for now, see update below), based on linux containers	Mac (on Mac hardware), Windows, Linux, etc. Relies on VirtualBox or VMWare.
Host OS	Mac/Win/Linux. Windows and OS X use boot2docker which is essentially a small linux virtualbox VM that runs the linux containers	Mac/Win/Linux
Configuration	Dockerfile describes steps to build the docker container, which holds everything needed to run the program. You can start from standard images downloaded from dockerhub such as Ubuntu, which is an extremely minimal ubuntu install, and you can upload your own images.	Vagrantfile describes what OS you want, any host<->guest file shares, and any provisioning scripts that should be run when the vm ("box") is started. You can start from standard fairly-minimal boxes for many OSes downloaded from Atlas, and upload your own box.
Building	When you build a docker image it creates a new container for each instruction in the Dockerfile and commits it to the image. Each step is cacheable so if you modify something in the Dockerfile, it only needs to re-execute from that point onward.	Many people use Vagrant without building their own box, but you can build your own. It's essentially a case of getting the VM running how vagrant likes it (packer can help here), making your customizations, then exporting and uploading.
Running	When you docker "run" something it creates a container from the image specified, and runs your application inside. All of the regular system stuff you might expect to be running (rsyslog, ntpd, sshd, cron) isn't. You have Ubuntu installed in there, but it isn't running Ubuntu. The "run" command allows you to specify shared folders with the host.	You run the full-blown OS inside a VM with all the bells and whistles (rsyslog, sshd, etc.) you would expect. When you "vagrant up" it brings up the VM and runs any provisioning scripts you specified in the Vagrantfile. This is typically where you do all the installing necessary to get your build environment ready. The provision script is the heart of the reproducible build you're creating. Once the provision script is done you'll typically have a Makefile or other scripts that will SSH into the environment and do the actual software building.
SSH access	If you're trying to ssh into your container, you're almost certainly doing it wrong.	SSH is a core part of the technology and used heavily. vagrant invisibly manages selecting ports for forwarding ssh to each of your VMs
Startup time	<1 second	<1 minute
Suitable for	Production and Dev/test	Dev/test only

Further reading

If you read this far you should also read this, where the authors of both tools explain the differences:

"Vagrant is a tool for managing virtual machines. Docker is a tool for building and deploying applications by packaging them into lightweight containers."

Update Aug 24, 2015

Microsoft has thrown their weight behind docker, and will be implementing shared-kernel containerization for Windows Server, so you will be able to run Windows Server containers on Windows Server as Mark explains here. You'll still need linux to run linux containers natively, and boot2docker will continue to be a way to bring linux containers to other OSes. Once container-capable windows servers are available from Microsoft/Amazon/Google clouds, much of the niche that Vagrant occupies will have been eroded, and windows server docker containers should be a better solution to the problem of being unable to share windows build VMs due to licensing.