The best choice I have found and use for all videos is in .mp4 format which in Vegas is internet hd 1080p then at the bottom set the average bit rate around 7-8,000,000 and then the max to like 10,000,000 you can go even lower for things like youtube, maybe even down to an average of 3,000,000 and selecting the two pass option can help keep the file size good and retaining quality.
Let me know if that makes sense