Step 1Place the sugar, water, orange juice and marmalade in a small saucepan. Bring to a boil. Cook until the mixture is syrupy, 3 to 4 minutes. Remove from the heat. Stir in thezest and vanilla.
Step 2To remove the rind from the oranges, cut a flat top and bottom on each orange. Set each orange on a cutting board. Use a sharp knife to cut the rind from each orange, working from top to bottom, slicing away all the rind and pith, going right to flesh. Cut the oranges crosswise into 1/3-inch thick slices. Place in a bowl. Add the syrup and coat the slices. Refrigerate the oranges at least 4 hours or overnight.
Step 3Serve the oranges chilled or at room temperature. Just before serving, thinly slice the mint leaves and sprinkle over the oranges.