I know that YouTube will re-encode it after I upload. I want re-encoding to happen at as few steps as possible. What's the ffmpeg command that will add the 3D metadata what YouTube wants without reencoding it at that step?
If you're still looking for a solution, I have good news. You don't need to reencode your file with ffmpeg! You can render an SBS 3D video right out of Vegas that has the required metadata for YouTube.