Preheat the oven to 400 degrees F.
Place 2 handfuls of rhubarb in a baking dish and roast for about 20 minutes until tender.
Meanwhile, in a medium pot over medium-high heat, bring the remaining rhubarb, water, and sugar to a boi. Reduce the heat, cover the pot, and simmer, stirring occasionally, for about 15 minutes until the rhubarb has almost melted in upon itself into a luscious, pink soup.
Stir in the vanilla and give it a taste--add a little sugar if it's too tart.
Add the roasted rhubarb and chill the soup.
To serve, spoon into bowls and add a scoop of vanilla ice cream.