Preheat oven to 250 degrees. Line a large baking sheet with parchment paper.
Beat egg whites and vinegar at high speed using an electric mixer until soft peaks form.
Add sugar gradually?one tablespoon at a time, mixing well after each. After all sugar has been added in then add vanilla and cornstarch. Mix until blended. The meringue should form stiff, shiny peaks when finished.
Spoon the meringue onto the center of parchment paper lined baking sheet forming a large mound about 7-8 inches in diameter. Bake for an hour until the outside is dry and slightly crispy but the inside is still be soft. Turn off oven, open the door a little and allow meringue to cool completely.
Transfer cooled meringue to a serving platter. Cover top of meringue with fat free cool whip and then add berries. Serve immediately.