I love Vagrant so was super happy to see this existed. Normally I use VMWare as the provider and this requires VirtualBox, which is clearly stated on the github page. So, I simply moved over to a machine with VirtualBox installed and BAM, one "vagrant up" later and I have WHMCS dev machine running.
Also, the provision scripts are a pretty good model to follow if you want to install a dev box by hand. All in all, 5 stars.