Stir together soy and vinegar in a bowl for dipping sauce.
Stir together chicken, egg, cilantro, scallions, sesame oil, and salt with a fork until just blended.
Spread cornstarch in a shallow baking pan. With wet hands form chicken mixture into 1-inch balls, transferring them to baking pan as formed. Wash and dry your hands, then gently roll balls in cornstarch until coated. (Balls will be soft.)
Heat 1/4 cup vegetable oil in a 12-inch heavy skillet over moderately high heat until hot but not smoking, then cook half of balls, turning over occasionally, until firm and golden (they will flatten slightly), 2 to 3 minutes. Transfer with a slotted spoon to paper towels to drain. Add remaining 1/4 cup oil to skillet and cook remaining balls in same manner. Serve meatballs with dipping sauce.
