Perhaps it's just that in retrospect, it seems a relatively small jump from heat pumps to vapor chambers.

Do you know if the vapor chambers operate at reduced internal atmospheric pressure? Unless I'm missing something, in order to get the liquid-gas phase boundary to a useful temperature, you'd have to bring the pressure down to, idk, 10kPa (boiling point of water is ~50C)? That would complicate manufacturing for sure, and also means that any leaks are catastrophic for your thermal solution.

Also I would be remiss if I did not note that the refrigerant designation of water is R-718.

> Do you know if the vapor chambers operate at reduced internal atmospheric pressure?

Indeed they do. A random search found this company that manufactures vapor chambers and they have a short discussion: https://radianheatsinks.com/vapor-chamber-heatsink/