vproto can parse most (or maybe all...) of the proto2 spec. It compiles the Steam Protobufs generating valid code and also parses the protoc plugin protobufs (which can be found in plugin/google/protobuf). It can parse options, extensions, map fields and extends definitions but does not generate relevent code for it.
vproto is used by vapor for both generating V code and runtime packing and unpacking of those generated protobufs