Finish stir fry: Heat second tablespoon oil in the wok over medium high heat. Add onion and cook, stirring occasionally, until softened, 5 to 7 minutes. Add tomatoes and cook for 1 minute. Add beef back to the wok with tomatoes and onions. Add 1 tablespoon soy sauce, ½ tablespoon vinegar, and salt and pepper. Cook for 1 more minute.