rsync should *always* be pre-installed in SmartOS (global and local zones), as it's part of the kernel. Previous commits incorrectly attributed #rsync_install to running rsync, rather than to installing the rsync command.
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.