I don't use Vegas 12 but here are some links that may help you with your question:
http://www.youtube.com/watch?v=muNDIzNrB1s
http://www.youtube.com/watch?v=qvUe7s660gU
http://www.youtube.com/watch?v=odQiXruP4UY
http://forums.creativecow.net/thread/24/948685
http://www.lcpdfr.com/topic/19084-sony-vegas-12-pro-youtube-render-settings/
http://filmmakeriq.com/2009/12/sony-vegas-rendering-in-1080p-for-youtube-hd/