Similar ends, but different means. Vagrant would launch a full-blown VM (likely on the order of several minutes), whereas fig'ing will start a container (on the order of a few seconds).
See linked page at https://docs.docker.com/installation/mac/
Note VirtualBox is one of the only modules blacklisted by the Linux kernel not for being proprietary (it's Open Source) but because it's terrible.
We agree that VBox is not ideal but it is free and we would like to have other options (Fusion, HyperV) available in the future as well.
For the sake of simplicity and distribution the OSX and Windows installers package boot2docker with Virtualbox.