Also, if you lower the heat, you're not bringing water to the surface fast enough to keep an even moisture gradient between the core and shell and again, this will cause damage. 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.