Here's an Ubuntu 14.04 Vagrantfile

This should fix vbguest issues in bug-1001567 as well.
Ah yes, the reason for this all comes back to me now (sorry for not documenting it as I went). Older versions of Vagrant don't support the implicit mapping a vagrantcloud box name to a url. If you were to use this Vagrantfile without the box URL on a 1.0 or older vagrant you'd get an error and no box would be downloaded.