Drain and set aside pineapple, reserving 1/4 cup of the juice. Whisk the reserved pineapple juice, vinegar, ketchup, soy sauce and sugar in a medium bowl until smooth. Place tofu in a large bowl; toss with 3 tablespoons of the sauce. Let marinate for at least 5 minutes and up to 30 minutes.
Meanwhile, add cornstarch to the remaining sauce and whisk until smooth.
Heat 1 tablespoon oil in a large nonstick skillet over medium-high heat. Transfer the tofu to the pan using a slotted spoon; whisk any remaining marinade into the bowl of reserved sauce. Cook the tofu, stirring every 1 to 2 minutes, until golden brown, 7 to 9 minutes total. Transfer to a plate.dd the remaining oil to the pan and heat over medium heat. Add garlic and ginger and cook, stirring constantly, until fragrant, about 30 seconds. Add red and green peppers and cook, stirring often, until just tender, 2 to 3 minutes. Pour in the reserved sauce and cook, stirring, until thickened, about 30 seconds. Add the tofu and pineapple and cook, stirring gently, until heated through, about 2 minutes more.