Slice the cored and peeled apples into eight wedges.
Open one of the cans of crescent rolls and carefully unroll it. Separate the 8 triangles from each other.
Place one apple slice on the wide end of the triangle, then roll up like a crescent roll (the package has an illustration on the back if you’re not sure what that should look like). Tuck the ends around the apple slice to completely enclose it. Place in the baking pan and repeat with the seven remaining triangles from the first can, then repeat again with the second can of crescent rolls.
Place the butter in a large microwave safe measuring cup and microwave until melted, about a minute.
Stir in the sugar, vanilla, and cinnamon, then spread evenly over the dumplings in the pan.
Pour the Mountain Dew around the edges of the pan.
Bake at 350F for 35-40 minutes, or until the crescent rolls are golden brown on top.