Heat the butter in a large saucepan over moderate heat. Add the onion, carrot, potato, salt and pepper and saute until the vegetables are tender, 7 to 10 minutes. Add the Swiss chard, stirring to mix well, cover the pan and simmer over low heat until chard is tender, 7 to 10 minutes. Add the vinegar and simmer, uncovered for a few minutes longer. Serve warm.