Preheat the oven to 190 C, gas Mark 5. Mix the rhubarb and vanilla sugar together in an ovenproof dish. Do not add water. Cover with foil and bake for 45 minutes to one hour , or until the fruit is completely soft.
Drain in a colander or sieve, reserving the juice. You should have about 500 ml. Pour the juice into a saucepan and bubble away until reduced by about half. Pour into a jug and leave to cool but do not refrigerate.
Puree the fruit until smooth then cool and chill.
Whip the cream in a large chilled bowl until very thick but not stiff. Carefully fold in the rhubarb puree, then some of the reduced juice. Put the remaining juice in a jug so people can add more. Serve the fool chilled in small bowls.