Preheat the oven to 350 degrees F. Spray a 9 x 13 pan with nonstick spray and set aside.
Stir together the lemon cake mix, eggs, butter, and vanilla extract until completely combined.
Spread a little over half of the prepared mix on the bottom of the pan - the mix just has to cover the pan.
Spoon the entire can of lemon pie filling evenly over the cake mix.
Using a cookie scoop or spoon, dollup small amounts of the remaining batter evenly over top of the pie filling.
Bake for 32-35 minutes or until golden brown on top or until a fork inserted into the top of the cake mix (not the center filling) comes out clean.
Allow to cool completely and then cut the bars into large squares.
Frosting
In a bowl, beat together the cream cheese and butter (do not melt the butter or cream cheese, just soften to room temperature) until smooth. Slowly add in the powdered sugar and the vanilla. Beat until completely smooth. Slowly add in the heavy cream to thin to desired consistency. (This is optional, the frosting works without the heavy cream, but adds creaminess and thins the frosting.)
Fill a frosting bag fitted with a #3 tip (or a plastic bag and cut off the tip) with frosting and pipe the frosting on to the bars. Alternatively you can spread the frosting on the bars with a knife.