In a medium bowl, whisk together the soy sauce, vinegar, sugar, and cornstarch. Set aside.
Heat the vegetable oil in a large skillet or wok over medium high heat, and place the steak strips. Cook for a few minutes until beef is browned. Season with salt and pepper. Remove from the pan and set aside.
In the same skillet, fry the onion until translucent. Add the red and green bell peppers and cook for about 3 minutes until softened.
Add the garlic and ginger and cook until fragrant.
Place the beef back in the skillet with the peppers and onions, and pour over the sauce. Cook, stirring for about 2 minutes, to combine.