Make the filling: Bring water, maple syrup, granulated sugar, quinces, and vanilla seeds and pod to a simmer in a large pot over medium heat. Cover with parchment, and cook until quinces are soft and rosy pink, about 2 hours. Discard vanilla pod.
Preheat oven to 375 degrees. Make the topping: Sift together flour, cornmeal, granulated sugar, baking powder, and 3/4 teaspoon salt, and then sift again. Cut in butter with a pastry cutter or rub in with your fingers until mixture resembles coarse meal with some large pieces remaining. Make a well in the center. Pour in cream; stir until combined.
Transfer quinces to a medium bowl using a slotted spoon. Add 1 cup poaching liquid and the cornstarch, and toss to combine. Pour quinces with juices into a 9-inch deep-dish pie plate. Arrange large spoonfuls of topping mixture around outer edge of pie, leaving a space in the center. Sprinkle almonds on top, and bake until liquid is bubbling and topping is golden, about 50 minutes. Let cool completely.
Make the maple cream: Whisk cream with a mixer on medium speed until stiff peaks form. Fold in maple syrup. Garnish pie with confectioners' sugar. Serve with maple cream.