For the cake: Preheat the oven to 350 degrees F. Spray a 9-by-13-inch cake pan with baking spray.
Combine the cake mix, butter, vanilla, eggs and 1/2 cup reserved mandarin juice in the bowl of an electric mixer and beat for 4 minutes on medium-high speed. Add 1 1/2 cups of the drained orange slices and beat again until the oranges have broken into small pieces, about 1 minute. Transfer the batter to the prepared cake pan and bake until golden brown and set, 25 to 27 minutes. Allow to cool completely, then turn out onto a large platter.
For the topping: Cream together the cream cheese and confectioners' sugar in the bowl of a stand mixer fitted with a paddle attachment, 1 to 2 minutes. Remove the mixture to a small bowl. Add the cream and vanilla to the stand mixer bowl and switch to the whisk attachment, then whip until soft peaks form, about 2 minutes. Add the cream cheese mixture, distributing it evenly around the bowl. Whip until stiff peaks form, 3 to 4 minutes. Fold in the pineapple.
Spread the frosting evenly over the cooled cake and refrigerate for at least 2 hours. Garnish with the remaining mandarin orange slices.