Cook first 3 ingredients in a Dutch oven over medium heat, stirring often, 35 minutes or until thickened. Remove from heat, and discard lemon. Add butter and vanilla; cool.
Fit 1 piecrust into a 9-inch pieplate according to package directions. Pour applesauce mixture into crust.
Roll remaining piecrust to press out fold lines, and cut into 1/2-inch strips. Arrange strips in a lattice design over filling; fold edges under, and crimp.
Bake on lowest oven rack at 425° for 30 to 35 minutes or until golden, shielding pie with aluminum foil to prevent excessive browning, if necessary. Serve with whipped cream, if desired.