Vagrant uses VirtualBox to handle creation of the actual virtual machines. Head over to https://www.virtualbox.org/wiki/Downloads to find the appropriate installer for your system.
If you're running Mac OS X, you must also install Xcode. Users running Lion can can install Xcode through the App Store. You can also grab the installer from http://developer.apple.com/xcode/.
Once you've got VirtualBox and Xcode installed, you're ready to install Vagrant. The official recommendation is to install Vagrant using builds from http://downloads.vagrantup.com/, so grab the latest and install it.
Also you'll probably need to install NFS on your computer to work with this project. Mac OS X has it by default, but running on Linux will require nfs-kernel-server installation.
NOTE: This project is intended to work with Vagrant v1.3.2
Now would be a good time to actually check out the project :
git clone http://github.com/bcwaldon/vagrant_devstack.git
The project has submodules for other recipes:
git submodule init
git submodule update
You can set up a local yaml-formatted config file to override the default settings used with the project. Place your config file at etc/vagrant.yaml or set a custom location in the environment variable VD_CONF. See a sample config at etc/vagrant.yaml.sample.
DevStack itself allows you to define a localrc file. This file is injected into your environment and sourced before the environment is built. You can use this to override settings such as MYSQL_PASSWORD or NOVA_REPO. See http://devstack.org for more information. If you decide to create your own localrc file, place it at etc/localrc file or set the VD_LOCALRC environment variable to its location.
At this point you can run vagrant up and ssh into your DevStack environment!