I've been looking at the same idea also. Using low pressure or other liquid that evaporates fast. Sure, in that case, I'm building a radiator. But... as it evaporates, and condenses, I'm also building a vapo chill system
Having that topick, I've been exploring vapo. (as that is what a closed bong is) Vapo chillers use an active condenser to change the pressure of the liquid, and that makes it possible to exchange the heat effective.
It is possible to build a "passive" condenser, but to make it any efective, it needs to be huge!
Look at destilling alcohol. With a setup like that, and actively air cooling the condensing spiral, it will work, and it will give you the below ambient temperatures.
The things to consider are the space needed, and the fact that you don't want the liquid used in your watercooling system

, as it starts to evaporate once it hits your CPU.
For me it was to expensive to experiment with, as I don't have the gear for alcohol destilling availible.