Just thought I'd add a little more info here since I finally got this to work the way I want it to...
I don't know what kind of hardware you have, but even though VP9 works in v23.2.1 it takes so much "horsepower" that you have to have an insanely fast machine to encode in vp9 in realtime at any usable resolution.
Unless you have the hardware, its best to stick with VP8. But you will run into issues there because it won't work either without some tweaking.
I've posted a work-around to get vp8 encoding for webm for the purposes of streaming video to icecast in another thread. Just search my posts.
Good luck, and happy streaming.
-Alan