Rub potatoes with oil. Bake in 400°F oven 45 minutes or until tender. When cool, cut each lengthwise into halves and each half into thirds. Scoop out potato flesh, leaving 1/2-inch thick layer of potato in each skin. Reduce oven temperature to 350°F.
While potatoes are baking, melt butter in a heavy skillet over medium heat. Add onions; cook until tender and golden brown, about 10 minutes. Stir in vinegar and sugars until dissolved.
Place potatoes, skin-side down on greased baking sheet; sprinkle with salt and pepper. Mix cheese and bacon. Sprinkle half of cheese mixture over potatoes; top with caramelized onions and remaining cheese mixture. Bake until skins are crisp and cheese has melted, about 20 minutes. Transfer to platter. Garnish with parsley and serve with sour cream, if desired.