Preheat the oven to 450 degrees F (230 degrees C).
Combine the flour, sugar, and cinnamon; reserve 1/4 of this mixture. Add the remaining 3/4 flour mixture to the elderberries and grapes, and mix thoroughly. Sprinkle the reserved flour mixture evenly over the bottom of the crust in pie plate. Turn the filling into the crust, and dot with butter.
Sprinkle the vinegar over the pie filling, and top with remaining crust.
Bake for 10 minutes. Reduce the heat to 350 degrees F (175 degrees C). Bake for 35 to 40 minutes, or until golden brown.