You'll find some boards that were dried too slowly and they will have wet cores. You will find a few that are just right. The fibers are already set. So, I believe that pressure swings in a vac kiln slow the drying and cause case hardening. Vapor pressure is a function of temperature.
Vaporization increases when ambient pressure is decreased.
A liquid boils when vapor pressure equals ambient pressure. If you're dealing with something like red oak, you're decreasing pressure down in those vessels. The point that I am trying to make is this. Most resins in wood will become hard when heated and then cooled very quickly. Controlling the phlem temp around the load to evaporate the moisture you are bringing to the surface of the wood seems more logical to me.