Preheat the oven to 180 degrees F. Place a sheet of parchment paper on a sheet pan. Draw a 9-inch circle on the paper, using a 9-inch plate as a guide, then turn the paper over so the circle is on the reverse side. (This way you won't get a pencil mark on the meringue.)
Place the egg whites and salt in the bowl of an electric mixer fitted with the whisk attachment. Beat the egg whites on high speed until firm, about 1 minute. With the mixer still on high, slowly add the sugar and beat until it makes firm, shiny peaks, about 2 minutes.
Remove the bowl from the mixer, sift the cornstarch onto the beaten egg whites, add the vinegar and vanilla, and fold in lightly with a rubber spatula. Place about half of the meringue into the circle on the parchment paper and smooth it within the circle, making a disk. Place the remaining meringue in a pastry bag fitted with a star tip and pipe the edge with kisses. If you have any meringue left over, pipe it into the middle of the disk and smooth it with a rubber spatula. Bake for 1 1/2 hours. Turn off the oven, keep the door closed, and allow the meringue to cool completely in the oven, about 1 hour. It will be crisp on the outside and soft inside.
Using a large metal spatula, carefully transfer the shell to a flat serving plate or cake stand. Fill the center of the shell with the whipped cream. Combine the strawberries, blueberries, raspberries and grapes in a bowl and toss with about 1/2 cup of raspberry sauce, or enough to coat the fruit lightly. Spoon the fruit carefully into the middle of the Pavlova and garnish with mint leaves. Serve immediately in large scoops with extra raspberry sauce.
Sweetened Whipped Cream:
Whip the cream in the bowl of an electric mixer fitted with a whisk attachment (you can also use a hand mixer). When it starts to thicken, add the sugar and vanilla and continue to beat until firm. Don't overbeat!
Place the raspberries, sugar and 1/4 cup water in a small saucepan. Bring to a boil, lower the heat, and simmer for 4 minutes. Pour the cooked raspberries, the jam, and framboise into the bowl of a food processor fitted with the steel blade and process until smooth. Chill.