Prepare oven and baking dish: Preheat oven to 350°F. Grease a 2-liter (11- x 7-inch) glass baking dish with butter.
Combine fruits: Drain and discard (or reserve for another use) the juice from the can of pineapple tidbits. Place pineapple pieces, cherry pie filling, and vanilla extract in a bowl; mix together. Pour mixture into prepared baking dish.
Add cake mix: Sprinkle the surface of the filling evenly with the yellow cake mix.
Melt butter and pour over cake mix: In the microwave, melt butter in a separate bowl and pour it evenly over the cake mix. Use a spoon to spread the butter across the mix.
Bake cake: Transfer baking dish to the middle rack of the oven. Bake until the top is golden brown in spots and the filling is bubbling, about 45 minutes. Allow to cool before garnishing with whipped cream.
