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 a 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. Pile the meringue into the middle of the circle on the parchment paper and smooth it within the circle, making a rough disk.
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 on the inside.
Invert the meringue disk onto a plate and spread the top completely with sweetened whipped cream.
Combine the strawberries, blueberries and raspberries in a bowl and toss with about 1/2 cup of raspberry sauce, or enough to coat the berries lightly. Spoon the berries carefully into the middle of the Pavlova, leaving a border of cream and meringue.
Serve immediately in large scoops with extra raspberry sauce.
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!
