The Internet Engineering Task Force (IETF) published VP8 Data Format and Decoding Guide as RFC 6386 in November 2011.
View the RTP Payload Format for VP8 Video Internet-Draft at ietf.org.
To comment on this document, post to the webm-discuss list with [RTP] in the subject line.
Once you've built the reference encoder (vpxenc), you may want to try some of the encode parameters we've collected for common use cases.
The syntax shown here can serve as a model for ffmpeg encodes.