Preheat oven to 350 degrees. Grease a 9x13 inch pan or line with nonstick foil or parchment.
Mix cake mix, egg and oil until well blended, then press firmly and evenly into the bottom of the prepared pan.
In a mixing bowl, beat softened cream cheese until smooth. Add sugar and beat until blended, then using the lowest speed of the mixer, beat in the eggs. Add the lemon juice, lemon zest and vanilla and mix on low until smooth.
Pour over prepared crust.
Bake for 25 to 30 minutes, then let cool completely. Chill for a few hours before serving.
Lift from pan and cut into bars.