Preheat the oven to 350. Grease an 8 X 10-inch baking dish with margarine and set aside. Peel the apples. Core the apple and cut each apple into 6 wedges.
Wrap each apple wedge in 1 piece of crescent roll, beginning at the wide end of the triangle and ending at the point. Place each roll in the greased baking dish, seam side down. Repeat with the remaining 15 pieces of the crescent roll (you will have 2 apple wedges leftover.
Place the stick of margarine and stick of butter in a saucepan over low heat. Add the sugar and stir until the margarine and butter melt (the sugar does not have to melt completely). Remove from heat and add the vanilla.
Pour the butter/sugar mixture over the rolls in the pan.
Pour the can of Mountain Dew over the butter/sugar mixture on top of the rolls.
Sprinkle the cinnamon sugar over the top of the rolls.
Bake the rolls for 40 minutes or until golden brown. If desired, serve the dumplings with vanilla ice cream, spooning up some of the sauce from the pan on top of the ice cream. Enjoy!