Preheat oven to 375°F. In a large mixing bowl, beat together softened cream cheese and sugar until smooth. Add eggs one at a time, mixing well. Add sour cream, lemon zest and vanilla, mixing until well combined. Pour into two 8-inch ready-made graham cracker pie shells. Bake for 10 minutes. Without removing cheesecakes from oven, reduce oven temperature to 250°F and bake for about 70 minutes (1 hour and 10 minutes). Cheesecake is done when toothpick comes up clean. Allow cheese cakes to cool. Meanwhile, make cheesecake topping. In a bowl, whisk or beat together sour cream, sugar, lemon and salt until smooth and creamy. When cheesecake has completely cooled, spread topping evenly over cakes. Bake at 350°F for 10 minutes. Refrigerate for several hours or overnight.