I know a guy who runs a large vacuum kiln. He manufactures pool cue blanks. This is around 20' long and about 4' sq. It opens in a clamshell fashion with a heavy rubber seal on the clamshell mating surfaces. Lots of clamps. Basically you layer heat tape in between each layer of wood and this is heated on a schedule. Big transformer to make the tape hot. Vacuum removes the free water. I think he can run a load through in a few days.
I understand this is the only way to dry 12/4 oak without checking. I have thought about building a small version to mess with but I'm not sure if I have the ambition at this time.
There is a Russian guy on one of the forums that has built a vacuum kiln that is as big as a railroad car.
I seem to remember that many woodworkers don't like vacuum dried wood, something about texture. I think it may open up the wood grain more than a conventional kiln.