Toss the pork with 1/2 tablespoon vinegar and a pinch of salt in a bowl. Mix the remaining 2 tablespoons vinegar, the soy sauce, cornstarch, ketchup, 3 tablespoons sugar, 1/3 cup water and 1/2 teaspoon salt in another bowl.
Heat 2 tablespoons peanut oil in a large skillet or wok over high heat. Add the pork and slowly stir until it turns mostly opaque, about 2 minutes. Remove the pork with a slotted spoon and transfer to a plate. Discard the oil and wipe out the skillet.
Heat the remaining 1 tablespoon peanut oil in the skillet, then stir-fry the garlic with a pinch each of salt and sugar, 15 seconds. Add the carrots and scallions and stir-fry until crisp-tender, 2 minutes. (Add a little water if the garlic starts to stick to the skillet.) Add the pork, snow peas and soy sauce mixture; stir until the pork is cooked through and the sauce is thickened, about 3 minutes.