> I've sometimes wondered if it would be practical to operate a steam heating system at a partial vacuum so that the condensation/vaporization could occur at a lower temperature

He actually talks about exactly this in the video! https://www.youtube.com/watch?v=TQB0KK2rxcw&t=573

"You'll be sending steam up into the building at temperatures, in some cases, as low as 150 degrees" (83 degrees C)