Preheat the oven according to the directions on the cake mix box.
In a bowl, mix together the white cake mix with the crushed pineapple (including the juice).
Pour the cake batter into a greased pan.
Bake the cake in the preheated oven until done.
While the cake is baking, prepare the pineapple mixture by combining granulated sugar, vanilla extract, and the remaining crushed pineapple in a saucepan.
Cook the pineapple mixture over low heat until the sugar dissolves and the mixture thickens slightly.
Once the cake is done, remove it from the oven and let it cool.
Spread the pineapple mixture on top of the cooled cake.
Serve and enjoy!