Yes, heating leftovers in vacuum-sealed bags in the microwave is absolutely fine. However, it is important to check if the bag is microwave-safe before you do. Snip off a little section of one side before microwaving to allow steam to escape. Furthermore, if you leave it in the microwave for too long, you will ruin the flavor and nutritional content of the food. It would be best only to microwave it for a couple of minutes.