Wash rhubarb, trim and cut into 1/2 inch slices. Combine with water and sugar and simmer until soft. Stir in vanilla. Blend cornstarch with a little cold water to make a smooth stiff paste. Stirring constantly, add to rhubarb and cook for 5 minutes, or until thick and clear. Pour into glass serving dish. Whip cream until frothy, add sugar and vanilla and whip until stiff. Pipe through pastry tube in decorative swirls on compote or cover top with spoonfuls of whipped cream. If you prefer to serve without whipped cream, you could serve with a little milk poured on each portion.