Here's an Ubuntu 14.04 Vagrantfile

This should fix vbguest issues in bug-1001567 as well.
Could be me being frustrated with provisioning VMs and not waiting to experiment (it takes me an hour+ to download updates and such on my slow internet). This method definitely worked but I'm happy to leave the leading 'dxr' off if you're able to test and are certain it won't clobber existing VMs.