Combine pork mince, egg, sake, mirin, 1 tablespoon soy sauce, spring onion, ginger and cabbage. Mix well.
Place 1 to 2 teaspoons of filling in a gyoza wrapper. Rub water around the edges, then fold over to make a semicircle. Take one side of the wrapper and make crimps along the edges for a decorative pattern (it should look like pleats of a skirt) and press along the edges to seal the two sides together. Ensure there isn't much excess air caught inside the dumpling. Repeat till all filling is used.
Heat the vegetable oil in a frying pan over medium high heat. Place dumplings in pan and fry till bottom is browned, about 3 to 5 minutes.
Once browned, add 120ml of water to the pan. Cover and cook 5 minutes.
Meanwhile, for the sauce, mix together the vinegar and soy sauce.