The patterns "all" is a special keyword that target all hosts in the inventory. Therefore it makes sense to accept "all:vars" as a group variable name. Note that "*:vars" pattern is not valid in an Ansible inventory. See http://docs.ansible.com/ansible/latest/intro_patterns.html#patterns Fix #7730
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.