Food is cooked in the pot on a regular stove at a high heat. After the food is fully cooked, the pot's lid is put on and the pot is inserted into the vacuum flask. The heavily insulated lid of the flask is closed and locked air-tight. The pot and food are left in the vacuum flask for several hours. The food continues cooking in its own heat, and stays warm.
The typical user prepares a meal in the morning, heats the meal in the pot, places the pot in the vacuum cooker and returns home after work to enjoy a hot meal. Normally, reheating is not required as the food remains hot enough for consumption after 6 to 8 hours.
The main advantages are carefree operation, and zero power consumption during the prolonged cooking process.
The main disadvantage is the risk of food poisoning as the food temperature slowly decreases to levels which may allow bacteria growth in the food. The danger of food poisoning can be reduced by thoroughly cooking the food at high temperatures before putting it in the vacuum flask.
Also, it is important to buy a vacuum cooker that seals and insulates well so the food temperature would be less likely to drop below the safe level.
Vacuum flasks appeal to Cantonese cooks because many Cantonese dishes require prolonged braising or simmering. When these cookers were first introduced in the US, they sold very quickly in the larger Asian supermarkets. The vacuum flask appears to be a modern replacement for the familiar crock pot, except that it needs no power.
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 insulates the pot, so the food in the pot remains hot over several hours.
Note that a different kind of vacuum cooker is used in the candy manufacturing industry to cook candies at low pressures. That is a different topic altogether.