Previously, failures in applying the puppet manifests would be ignored, because puppet apply/agent don't have any useful exit codes by default. (Errors are printed, but vagrant continues.) Use the option --detailed-exitcodes of puppet apply/agent to check for success.
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.