In some cases the E1000 NIC type is the only acceptable value. Since defaulting causes breakages to existing boxes, leave the default value as `nil` but check the VirtualBox version in use and print warning to user if VirtualBox version is vulnerable and E1000 NIC types are configured for use within defined network adapters.
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.