lsb_release command is included in lsb-release package. Sometimes this is not installed; ex: Docker official Ubuntu image. /etc/os-release is included in base-files package, so this is more reliable than lsb_release.
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.