Preheat your oven to 350°F (175°C).
In a small bowl, mix together the sugar and cinnamon. Set aside.
Unroll the crescent roll dough and separate it into triangles.
Place one apple slice on the wide end of each triangle and sprinkle with a little cinnamon sugar. Roll the dough around the apple slice, starting from the wide end and rolling to the point. Pinch the edges to seal.
Place the rolled dumplings in a greased 9x13 inch baking dish.
In another bowl, combine the melted butter, apple juice, and vanilla extract. Pour this mixture evenly over the dumplings in the baking dish.
Sprinkle any remaining cinnamon sugar over the top of the dumplings.
Bake in the preheated oven for 30-35 minutes, or until the dumplings are golden brown and bubbly.
Allow to cool slightly before serving. Enjoy warm, optionally with a scoop of vanilla ice cream.