Does the MP4 HTML5 compatible video type allow for mono audio?
The Mainconcept or Sony codecs in Vegas are subpar for low-bitrate output such as needed for HTML5 use. Use ffmpeg to generate your HTML5 videos. Yes, mono audio is acceptable for HTML5 use, but once you use ffmpeg/x264 for generating those videos, the 6kB/s savings you get from switching to mono audio won't matter much if at all.

Get the 32-bit static build of ffmpeg from here. Render a high bitrate Mainconcept MP4 from Vegas, say, 12 Mbps. And then run the following ffmpeg command:

ffmpeg -i "vegas-output.mp4" -crf 23 -c:a aac -b:a 96k -strict -2 -movflags +faststart "html5-video.mp4"


If you still want mono audio, insert

-ac 1


after aac.

If you don't like the quality or size is too large, then play around with the CRF value. They go from 0 to 63 with lower being better. 18 to 28 is the usual range you want to stick with.