Vacuum flask cooking is a recent invention introduced in the Asian market in the mid 1990's. The vacuum cooker is made of a stainless steel vacuum flask typically 12 inches in diameter and 10 inches tall. They comes in various sizes ranging from 8 inches to 16 inches. Inside the vacuum flask is a removable pot with handle and lid. Food is cooked using the inner pot on regular stove in high heat. After the food is fully cooked, the lid is put on and the whole pot is inserted into the vacuum cooker. The heavily insulted lid of the flask is closed and locked air tight. The pot and food is left in the vacuum cooker for the next few hours. Without external heat applied, the food basically continued cooking itself with the residual heat.
Note that the food is NOT cooked in a vacuum. It is cooked inside a vacuum flask. The vacuum in the wall of the cooker is just for extremely high insulation so that the pot inside can be kept hot over several hours. Note that a different kind of vacuum cooker is used in the candy manufacturing industry where the candies are cooked in a low pressure environment. That is a different topic all together.
The typical user would prepare something in the morning, put it in the vacuum cooker and return home after work to enjoy a hot meal. Normally reheating is not required because the food would still be hot enough for consumption after 6 to 8 hours. The main advantages are carefree operations, and most attractively of all, the zero power consumption during the prolonged vacuum cooking process. The main disadvantage is the risk of food poisoning as the food temperature slowly decreases to the level which may allow bacteria growth inside the cooker. However, the danger can easily be eliminated by thoroughly cooking the food to a high temperature before putting it in the vacuum flask. Also it is important to buy a good brand that seals and insulates well so that the food temperature would never drop below the safe level.
These vacuum cookers are especially appealing to Cantonese cooks because many Cantonese dishes require prolonged braising or simmering. When these cookers are first introduced in the US, they were selling like hot cakes in larger Asian supermarkets. Basically the vacuum cooker replaces the familiar crock pot with the only exception that you don't even need to plug it in. external link: an example of a vacuum cooker