Peel potatoes and cut into 1/4" slices. Place in a 12x8 baking dish. Slice lemon and oranges very thinly and place slices on top of sweet potatoes. Combine melted butter, sugar, orange juice and cinnamon. Stir in vanilla. Pour mixture over sweet potato mixture, cover with plastic and let stand 1 hour. Bake, uncovered, at 350* for 1 1/2 hours, basting often with pan juices, or until potatoes are tender. Makes 12 servings.