Position a rack in the lower third of the oven and preheat to 250 degrees . Line a large baking sheet with parchment and outline a 10-inch circle on the parchment. Using an electric mixer with the whisk attachment, beat the egg whites and salt at medium speed until soft peaks form, about 3 minutes. In a small bowl, stir together 1 tablespoon sugar and the cornstarch.
With the mixer at medium-high speed, slowly add 1 1/2 cups sugar until the whites are stiff and glossy, about 5 minutes. Turn off the machine and, using a spatula, fold in the vinegar and the cornstarch mixture.
Spoon dollops of the meringue onto the parchment to evenly fill the circle. With the spoon, make a shallow hollow in the center for the filling, about 1 inch deep and 4 inches wide. Bake the meringue for 45 minutes, then lower the temperature to 200 degrees and bake for 1 hour longer. Turn off the heat, set the oven door ajar and let the meringue sit in the oven for 30 minutes more. Remove the meringue from the oven and let cool completely on the pan, then, using a large metal spatula, loosen from the parchment and slide onto a large platter. Reserve any bits of crumbled meringue for garnish.
Meanwhile, in the clean mixer with the whisk attachment, beat the cream, vanilla and remaining 3 tablespoons sugar at medium-high speed until stiff peaks form. Cover the top and sides of the meringue with two-thirds of the whipped cream. Spoon the mango and passion fruit into the hollow, then cover with the remaining whipped cream. Sprinkle the reserved meringue crumbs on top. Serve within an hour.