Place 2 cups of water and the sugar, cinnamon, and vanilla bean in a medium, nonreactive saucepan and bring to a boil.
Add the rhubarb and prunes and simmer over low heat until the rhubarb is just tender, 7-10 minutes. Don't allow it to get too soft - it will "cook" further while macerating.
Using a slotted spoon, remove the rhubarb and prunes and transfer to a large attractive serving bowl.
Slice the oranges into thin rounds (if they break apart into little sections after you slice them, that's perfectly fine), and add them, along with the raspberries, to the bowl.
Boil the syrup remaining in the saucepan over moderately high heat until reduced by about half.
Remove the cinnamon and vanilla bean (you can dry the bean and save it for another use, like burying it in a bowl of sugar to prepare vanilla sugar) and pour the hot syrup over the fruit.
Stir well. Let the fruit cool to room temperature, then cover and refrigerate for several hours.
Garnish the compote, if you'd like, with fresh mint leaves.