Preheat oven to 375 degrees F.
Split any larger stalks of rhubarb in half. Cut the rhubarb into 1/2-inch slices and place in a mixing bowl. Puree one half-pint of the raspberries with the orange juice and strain over the rhubarb. Add the vanilla beans scrapings, sugar, and cornstarch and toss well. Place in an 8 by 8-inch baking dish and bake for 30 minutes, carefully stirring half way through. Lower oven temperature to 350 degrees F.
Line a large sheet pan with parchment paper. Place 8 (4-inch) flan rings on the pan. Place 4 sheets of phyllo on a work surface horizontally, cut them in half from top to bottom, and cover with a barely damp towel. Carefully brush the top sheet with melted butter, sprinkle it with sugar, gather it up, and place in the ring. Repeat with the remaining pieces.
Bake 10 to 12 minutes, until the phyllo is golden brown. Let the phyllo cool in the rings, then use immediately, or place in an airtight container to store overnight.
If serving immediately, fold in the remaining half-pint of whole raspberries to the filling and sandwich it between the disks of phyllo. Or let cool and reheat the filling up in a microwave and fold in the whole raspberries. Sprinkle tops of the dessert with powdered sugar.