This separates the truly basic pieces of Chef (like install url and log_level) from the runner pieces of Chef (like provisioning_path). This is necessary because the Chef Apply provisioner does not actually need most of the Chef configuration options.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.