Prepare the meringue: Heat oven to 300°F/ 149°C °F. Line a baking sheet with parchment paper and set aside.
In the bowl of a stand mixer, or using a hand mixer, combine the egg whites and salt. Beat at slow speed, gradually increasing to high. Continue until soft peaks begin to form.
Gradually add the ¾ cup sugar, one tablespoon at a time, making sure the sugar is dissolved before adding the next spoonful. Keep beating until stiff, glossy peaks form and all the sugar has dissolved. To check, rub a little meringue between your fingers. If you don't feel any gritty sugar, it's ready.
In a small bowl, whisk the cornstarch and vinegar until combined. Gently fold the mixture into the meringue along with the sugar until just combined.
Using a spoon, scoop the meringue onto the prepared baking sheet, shaping it into 6 small disks about 3 inches in diameter. Don't worry about making them perfect; just try to keep them roughly the same size so they bake evenly.
Place the meringues in the oven and immediately reduce the temperature to 275°F/ 135°C. Bake for 1 hour.
Turn off the oven and allow the meringues to cool completely in the oven, for at least 2 hours. Do not open the oven door while they cool.
Prepare the peaches: Once the pavlovas have cooled completely, remove them from the oven and preheat it to 350°F/ 175°C. Line a baking sheet with aluminum foil or parchment paper.
In a medium bowl, add the peaches, vanilla, and sugar. Toss to coat. Arrange the peaches cut-side up in a single layer on the prepared baking sheet.
Bake the peaches, uncovered, until cooked through and fork-tender but still holding their shape, 20 to 30 minutes.
Make the whipped cream: Add the heavy cream and vanilla to a medium bowl. Using a hand mixer, whip the cream on medium-high until soft peaks form, about 3 minutes.
Assemble: Top the cooled pavlovas with whipped cream and roasted peaches and its juices. Serve immediately.