When preparing the NFS settings on VirtualBox the guest IP addresses are pulled from VirtualBox directly and any static addresses are pulled as well. This can lead to aquiring a host IP and machine IP but results in a failure of NFS mount because the IPs are not on the same network. This filters the machine IP result to validate it is within the host adapter IP range.
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.