I didn't find much reference to this online, and it hung me up for quite some time: The Ansible plugin for Vagrant has changed fairly substantially between version 1.2.2 (which is the version currently packaged by debian testing/jessie) and version 1.4.0 (which is covered by the online docs).
The top things I've noticed are:
- No inventory file is automatically generated.
- The inventory_path option in 1.4.0 was called inventory_file in 1.2.2.
Since the documentation for Vagrant is not too comprehensive to begin with, I think it's best to just ignore it and read the source. For comparison, source of config.rb at 1.4.0 vs config.rb at 1.2.2.