1) Preheat oven to 350 degrees. Spray a 9x13 baking dish with nonstick cooking spray.
2) For the cake, combine cake mix, butter, 1/2 cup juice from the mandarin oranges, eggs, and vanilla. Beat for 3-4 minutes on medium-high.
3) Add drained oranges and stir until combined.
4) Pour batter into prepared baking dish and bake for about 20-25 minutes.
5) Remove from oven and cool completely.
6) Once cake is cool, blend juice from drained pineapples with the vanilla pudding mix.
7) Stir in powdered sugar. Then mix in whipped topping.
8) Finally, stir in drained pineapple.
9) Spread frosting on cooled cake. Optional: Top with slices of mandarin oranges.