Combine the pork with 1 tablespoon of the cornstarch in a medium bowl; toss well to soat and set aside. Combine the remaining 1 tablespoon cornstarch, the water, vinegar, sugar, ketchup, and soy sauce in a small bowl; set aside.
Heat a nonstick wok or a large, deep skillet ower medium-high heat until a drop of water sizzles. Swirl in the oil, then add the pork. Stir-fry until almost cooked through, 2-3 minutes. Add the ginger and garlic. Stir-fry until fragrant, about 30 seconds. Add the bell pepper and pineapple; stir-fry until crisp-tender, about 3 minutes. Add the vinegar mixture and cook, stirring constantly, until the mixture boils and thickens and the pork is cooked just through, 1-2 minutes.