Prepare the Beef: In a small bowl, combine the flank steak, cornstarch, salt, and pepper. Toss to coat. Then add the rice wine vinegar.
Make the Sauce: In a small bowl, whisk together all ingredients: the soy sauce, beef broth, rice wine vinegar, ginger, garlic, brown sugar, chili garlic sauce, and cornstarch.
Cook the Beef: Heat the vegetable oil in a large skillet or wok over high heat. Cook the beef in batches until browned and slightly cooked through.
Sauté the Vegetables: Add the red bell pepper, carrot, and snow peas to a seperate pan and cook until tender-crisp, about 3-4 minutes. Pour 1/4 C (or desired amount) of the sauce over the veggies.
Combine and Cook: Return all the beef to the pan. Pour the sauce over the beef and bring to a boil. Reduce heat and simmer until the sauce has thickened and the vegetables are coated, about 2-3 minutes.
Serve: Serve immediately over rice or noodles. Garnish with green onions, if desired.