Elevated command line is now rendered to a script which is uploaded to the guest and executed. This allows the command line itself to be less than 100 chars to start the script and any user commands are puts into the script which has unlimited* length.
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.