Position an oven rack in the center of the oven. Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper and use a pencil to trace a 12-inch round on the paper.
In a medium bowl, whisk the sugar and cornstarch until smooth.
In a stand mixer fitted with the whisk attachment, beat the egg whites on high speed until they begin to froth and turn opaque. Reduce the speed to low and slowly add the sugar mixture in 1/2-cup increments. Immediately add the vinegar, return to high speed, and beat until stiff peaks form, at least 2 minutes.
Pour the boiling water into the egg whites all at once. The egg whites will swell up. Beat the egg whites until the water is totally incorporated, stopping once to scrape the sides of the bowl. Add the vanilla and continue to beat until the egg whites form stiff, glossy peaks, 3 to 5 minutes.
Reach for a rubber spatula and carefully fill in the base of the traced circle with some of the meringue. Continue to add the meringue, fashioning fancy swoops and swirls, until you to have a thick layer that sort of resembles a cake. Bake the meringue for 10 minutes. Then reduce the oven temperature to 200°F (93°C) and bake for 40 minutes more.
Remove the meringue from the oven, place the baking sheet on a wire rack, and let it cool completely. Once cool, gently remove the parchment paper by rolling it out from under the meringue. Take care as the meringue will be very delicate.