Place a rack in the middle of the oven and preheat the oven to 250°F. Line a large baking sheet with parchment paper. In a small bowl, rub the vanilla bean seeds into the sugar and stir in the cornstarch. Sift to remove clumps.
In the bowl of a stand mixer fitted with a whisk attachment, whip the egg whites and salt, starting on low, increasing incrementally to medium speed until the egg whites turn opaque and white.
Increase the speed to medium-high, and slowly and gradually sprinkle in the sugar-cornstarch mixture. A few minutes after the sugar is added, pour in the vinegar. Increase the speed to high and whip until the meringue is glossy and stiff, about 5 minutes.
Spoon the meringue into 6 mounds on the parchmentlined baking sheet. With the back of a spoon, create an indentation in the middle of each for holding the filling once the meringue is baked. Place the baking sheet in the oven and bake for 70–80 minutes, or until the meringue is crisp and dry to the touch on the outside. The interior should have a marshmallow-like consistency.
Check the pavlovas at least once during the baking time. If it appears to be taking on color or cracking, reduce the temperature by 25 degrees, and rotate the pan. Once baked, gently lift the meringue from the baking sheet and cool on a wire rack.
Whip together the heavy cream, Greek yogurt and sugar in a mixing bowl until you have medium-stiff peaks. Spoon the whipped yogurt over the cooled meringue.
Push one pint of the raspberries through a fine-mesh strainer to remove the seeds and create a purée. Toss the rest of the raspberries with the purée.
To assemble the pavlovas, spoon the whipped yogurt into the wells of the cooled meringues, and then top with spoonfuls of the raspberry mixture. Serve immediately.