In a heavy 2 1/2- to 3-quart heavy pot combine rhubarb, sugar, and water and bring to a boil, covered, over medium heat, stirring occasionally. Reduce heat, and simmer, covered, stirring occasionally, until rhubarb begins to break down, 3 to 5 minutes.
Remove lid and briskly simmer, stirring frequently, until rhubarb is completely broken down into a thick purée and reduced to about 2 cups, about 20 minutes.
Remove from heat and stir in vanilla.
Transfer rhubarb to a bowl and let it cool, then chill, covered, until cold, about 1 hour. DO AHEAD: Purée can be made and refrigerated, covered, up to 2 days ahead.
Beat cream in a deep bowl with an electric mixer until it holds soft peaks—for this fool, I like the cream to be slightly beyond the soft peak stage but not quite stiff—and fold it into the rhubarb. Don’t feel you have to completely incorporate the cream and the rhubarb into a smooth pink fluff. I like to leave it a little streaky with swirls of rhubarb coursing through the cream.
Divide the fool among 6 goblets.
The fool can be made a day ahead and refrigerated, covered.
Let it stand at room temperature 20 to 30 minutes to soften slightly.
