Vacuum distillation is distillation performed under reduced pressure (vacuum), often in Rotary Evaporators.
Distillers chose this method for three main reasons: the purification of compounds not readily distilled at normal pressures, to lower the overall temperature for flavour preferences, or simply to save time or energy.
On a smaller scale, vacuum distillation is thought of as an ideal solution to capture elements that decompose when heated at atmospheric pressure (e.g. floral notes that would stew in a big copper pot). For the most industrial sized continuous stills, the combination of energy efficiency and the ability to refine at a level not possible under normal pressure tends to be why some producers adapt their stills to work under vacuum.