Have a look at the WebRTC google group topics filtered with video codec.
Because I wrote my bachelor thesis about WebRTC & SIP I know that WebRTC supported video codec isn't only VP8. It pepends on the codec availibility at the clients (for example two browsers).
The process of Session Description Protocol (SDP) exchange takes care of the finally used media codecs.
Currently there isn't a lot of documentation or litature of WebRTC. I got the most information out of different RFCs at the IETF.