Everywhere I look I read that a vagrant box is just a base "template" which vagrant builds off of. And that once you run "vagrant up" it actually creates a copy of the vagrant box so it should be safe to delete the box file once you run vagrant up and you'll still be able to use that virtual machine.
However this doesn't seem to work in practice. I have a base box called ~/.vagrant.d/boxes/precise32 and I created a vagrant file with that box name in the file and ran vagrant up. Now when I rename the file to ~/.vagrant.d/boxes/precise32.old and try to run "vagrant up" again I get an error:
* The box 'precise32' could not be found.
So it appears that the base box is not a template but in fact the live disk image that needs to be kept around. So why does all the documentation say otherwise, e.g. http://docs-v1.vagrantup.com/v1/docs/getting-started/boxes.html
"Each project uses the box as a base only, so once the project VM is created, modifications can be made without affecting other projects which may use the same box."