Peel potatoes. Cut potatoes, orange and lemon into 1/4" slices. Place potato slices in a lightly buttered 11 x 7 dish. Arrange lemon and orange slices over potatoes. Combine butter and next 3 ingredients. Stir in vanilla and pour over potato mixture. Cover with plastic wrap and let stand for 1 hour. Bake uncovered at 350 for 1 1/2 hours, basting often with pan juices. Serves 8 to 10.