Combine egg yolks, water, lemon juice, sugar, and cornstarch; mix well. cook mixture over medium heat stirring constantly until mixture thickens. Remove from heat; stir in butter, vinegar. Pour immediately into pie pastry.
In large bowl, beat egg whites, vanilla and cream of tartar with electric mixer until foamy. Gradually add sugar and salt, beating until mixture forms stiff peaks. you want your egg whites to be stiff, but not dry. Do not overbeat.
Spread meringue over hot filling, sealing to edge of pastry. Bake at 425 degrees for 5 to 7 minutes or until lightly browned. Cool completely before serving.