Preheat oven to 250 degrees. Take a piece of parchment paper and trace the shape of your pavlova on the paper – use an 8-inch round cake pan for a circle, or an 8-inch square baking pan for a square – or create a freeform design. Reverse parchment and place on baking sheet. (You will see your drawing through the parchment.)
Beat egg whites and pinch salt with a wire whisk until soft peaks form. Gradually add sugar, beating at high speed, until shiny, stiff peaks form. The sugar should be fully dissolved. Beat in vinegar and vanilla and fold in cornstarch. Spread meringue inside outline on parchment paper. Bake for 60 minutes, then turn off oven and leave in with door ajar until cool.
Using a spatula, carefully remove meringue to serving plate.
Meanwhile, rinse and hull strawberries and cut in half (if small) or thirds (if large). Rinse blueberries.
Using chilled bowl and beaters, beat whipping cream and vanilla until soft peaks form. Spread carefully on meringue. Place fruits on top. Serve immediately.