Remove stems from swiss chard. Chop leaves and slice stems, then set aside. In a large pot or skillet with a lid, cook bacon until crispy. Remove bacon and most of the fat, leaving about 1 tablespoon in pan. Chop bacon and set aside. Add butter to bacon fat in pan and sauté onions and chard stems until soft, about 5 minutes. Add greens, vinegar, and 1 cup water. Cover and cook for 5 minutes. Stir in chopped bacon and season with salt and pepper.