Heat 2 tablespoons oil in a 12-inch heavy skillet over moderately high heat until hot but not smoking, then sauté onion with salt, stirring occasionally, until golden brown, about 8 minutes.
Transfer to a bowl.
Pat pork chops dry and season with salt and pepper.
Heat remaining tablespoon oil in skillet over high heat until just smoking, then sauté chops until browned and just cooked through, about 3 minutes on each side.
Transfer chops to a platter and keep warm, covered.
Return onion to skillet and add tomatoes, then sauté over moderately high heat, stirring, until tomatoes are slightly wilted, about 2 minutes.
Remove from heat and stir in vinegar and salt and pepper to taste.
