Preheat oven to 350°F (*or 325º F for dark coated pans). Grease and lightly flour two 8 or 9-inch round baking pans. Which ever you prefer.
In a mixing bowl, beat cake mix, eggs, oil, mandarin oranges plus the juice (*reserving a few slices to garnish top of cake), and vanilla on low speed just until moistened. Then beat on medium speed for 2 minutes.
Pour batter into prepared baking pans and bake according to boxed mix directions, or until toothpick inserted in center of cakes comes out clean. Cool slightly, 10 minutes, then remove cakes to wire rack to cool completely.
For the Frosting: In a bowl, stir instant pudding mix into the pineapple with the juice until mixed, let rest 5 minutes. Fold in thawed whipped topping until well combined. Place one cooled cake onto a cake plate or stand. Add a layer of frosting to top, then place second cake on top of first layer, and spread remaining frosting over that top and sides of cake.
Chill in refrigerator for several hours or overnight before serving. (*I always place mine into a plastic cake keeper so that it doesn't dry out in any way.)
These also make wonderful cupcakes! Just follow the baking times on the mix that you choose to use.
