I'm really eager to use Vagrant to easily set up virtual machines for testing purposes. However I'm using KVM with libvirt and virt-manager. Although I think VirtualBox is nice software, I'm not feeling comfortable with the license of the VirtualBox extension. I'm looking forward to Vagrant supporting libvirt.
In the mean time I wanted to automatically install Debian machines. I have experience with FAI, and am using it at work. I have known about preseeding the Debian installer but have never used it.