In the VM config file port forward information is captured in the Snapshot defition as well as the VM definition. This causes duplicates in port forward entries if the file is queried for all available network adapters. Querying the VM config file for the the Network Adapters only part of the Machine definition resolves this and yields the most up to date port forward info.
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.
Generate proto
grpc_tools_ruby_protoc -I . --ruby_out=gen/plugin --grpc_out=gen/plugin ./plugin_server.proto