In bowl, combine cracker crumbs, sugar and melted butter till combined. Press into bottom and a little up the sides of a 10 inch spring form pan. Bake in a 325 degree oven 10 minutes; set aside. For the filling, beat cream cheese with mixer till smooth with sugar. Add zest and juice; beat till smooth. Beat in eggs, one at a time till completely incorporated; add vanilla. Bake in a 325 degree oven for an hour and fifteen minutes to an hour and a half, or till filling is nearly set in the middle of cheesecake when shaken gently. For topping, beat sour cream and sugar with vanilla till smooth with spoon. Then carefully spread evenly over the baked cheesecake. Bake in a 325 degree oven ten to fifteen minutes, or till topping is nicely baked with cheesecake. Cool fifteen minutes, then cut around edges with sharp knife, cool thirty minutes and remove from spring form. When completely cooled cover with plastic wrap and chill 24 hours before serving. Chill leftovers.