Cook bacon in a large heavy skillet over moderate heat, stirring occasionally, until golden. Add onion and cook, stirring occasionally, until golden. Stir in cabbage and salt to taste and cook, covered, stirring occasionally, 10 minutes, or until cabbage is very tender and beginning to brown. Stir in vinegar and 1 tablespoon parsley. While cabbage is cooking, heat butter in a 12-inch nonstick skillet over moderate heat until foam subsides, then cook pierogi until bottoms are golden. Turn pierogi over and add water to skillet. Simmer until water is evaporated and bottoms are golden, about 2 minutes. (If pierogi are large, cook in 2 batches, using half of butter and water for each batch.) Stir in remaining tablespoon parsley. Serve potato pierogi over cabbage mixture.