For vagrant-libvirt, the second disk can be added w/ libvirt.storage :file, :size => '5G'.
To make use of this disk, we can use docker-storage-setup, as described in http://www.projectatomic.io/docs/docker-storage-recommendation.
The lines in the config.vm.provision section below add DEVS="/dev/vdb" to /etc/sysconfig/docker-storage-setup. The commented-out line is supposed to run docker-storage-setup to set up the storage, but this isn't working when run from the Vagrantfile here. Once logged in to the vagrant VM, however, running sudo docker-storage-setup works as expected, except for a syntax error with part of the setup script which doesn't appear to stop the script from doing its job.
Vagrant.configure("2") do |config|
config.vm.box = "f22atomic"