The place where I worked we vacuumed dried apples and apple pieces, like French fry shaped. We first dried the apples down to 23% moisture (leathery), then spread them on a tray about 4' by 4', 2 inches deep, and put about 30 trays in the vacuum on shelves. The shelves were hollow and hot water circulated inside the shelves. Then we'd shut the doors on the vacuum, and pull a vacuum while heating. The heat combined with the vacuum puffed the apples up, and when they came out they were dry and brittle, at about 4% moisture. They snapped if you tried to bend them. Very tasty.
I suppose freeze drying would be similar but take longer to sublimate off the water.