What is the syntax for adding a locally existing VirtualBox .vdi to vagrant?
There doesn't appear to be an example of alternate URI schemes in the documentation.
Can it also use ssh and httpauth locations?
You cannot directly "import" a VirtualBox VM into Vagrant.
Basically you'll have to package the existing VirtualBox VM to a box
and then add it to vagrant.
NOTE: a box is tarred VirtualBox Exports (could be gzipped).
You would be doing
vagrant package --base mybox --output /path/to/mybox.box
See more here =>
http://docs.vagrantup.com/v2/cli/package.html
vagrant box add mybox /path/to/mybox.box