Vacuum drying means drying while pulling a vacuum. Put the material in a vacuum and heat it; once the pressure drops, water turns to vapor and escapes more easily, so it dries fast even at low temperature.
Benefits: low temperature protects heat-sensitive substances; low oxygen slows oxidation; the vacuum lets volatiles leave faster. It suits samples that are volatile at room temperature or that fear heat.
Principle
By the Clausius-Clapeyron relation, lowering pressure lowers a liquid's boiling point. Under vacuum, water vaporizes well below 100 °C; with added heat the drying is faster than in an atmospheric oven and avoids high-temperature degradation.
Relation to reduced-pressure drying
Vacuum drying and reduced-pressure drying are essentially the same (both dry below atmospheric pressure); the names emphasize different angles, vacuum stresses the vacuum drawn, reduced pressure stresses the lowered pressure. The DCT-A supports both atmospheric and reduced-pressure modes.