Preheat the oven to 400°. In a medium saucepan, combine the water, Armagnac, sugar and vanilla bean and bring to a simmer. Add the prunes and poach over moderately low heat until plumped, 15 to 20 minutes. Remove the prunes to a plate. Add the cranberries to the poaching liquid and cook until plumped, about 5 minutes. Remove to a plate. Simmer the poaching liquid until it reduces to a syrup, about 3 minutes. Strain the syrup.