In order to chill vegetables, you simply need a refrigeration system. So why not just put them in a giant refrigerator? Because a regular refrigerator would take too long to chill large quantities of vegetables. The produce would begin to rot before it ever gets to the store.

Vacuum cooling involves chilling the vegetables in a depressurized chamber. By lowering the pressure inside the chamber the air can cool much faster, and thus chill the vegetables much faster.

Why does decreasing air pressure allow the air to cool faster? Because the lower the air pressure, the lower the boiling point of water. Heat can be removed much faster in depressurized, dry air.