Unroll piecrust according to package directions. Fit into 9" pie plate. Fold edges under and flute. Prick bottom and sides with a fork. Bake at 450 degrees until golden. Remove from oven and let cool on a wire rack. Reduce oven temperature to 350°.
Whisk together sour cream and egg yolks in a medium saucepan; stir in lemon pudding mix, milk and lemonade concentrate. Cook over medium heat, whisking constantly, until thickened. Reduce heat to low and cook 2 minutes or until very thick. Remove from heat; cover and keep hot.
Beat egg whites, cream of tartar and vanilla at high speed with an electric mixer until foamy. Beat in sugar, one tablespoon at a time, until stiff peaks form. Pour hot filling into prepared crust. Dollop meringue onto hot filling. Lightly spread dollops together n decorative swirls, completely sealing meringue to pie-crust.
Bake at 325° for 22 to 25 minutes or until golden. Cool completely on wire rack.