This seems to be a very useful tool, one question though, what was the rationale behind choosing VirtualBox, have you done any benchmarks against Xen or Kvm or other hypervisors
VirtualBox was chosen since it is an open source, free choice which is available on Windows, Mac, Linux, and Solaris. It is simply the easiest way for developers to get started with a virtualized development environment.
Neither John nor I are familiar with any other virtualization platform, but making Vagrant modular enough such that this is possible is definitely a good idea for the future.
