1. Position rack in middle of oven and preheat to 350ºF. Line a baking sheet with parchment. Trace an 8-inch circle on parchment, then flip parchment over.
2. In a perfectly clean, dry bowl with clean, dry beaters, whip egg whites on medium speed until foamy, about 1 1/2 minutes. Increase speed to high; continue to whip, stopping and adding sugar in spoonfuls and scraping down sides of bowl, until mixture is stiff and glossy, about 5 minutes. Fold in vinegar and vanilla.
3. Use a spatula to spread meringue into an 8-inch circle on parchment, making a shallow well about 5 inches in diameter in center. Reduce oven temperature to 200°F. Bake until outside of meringue is lightly crisp and just barely browned but inside is still soft, 1 hour 30 minutes to 1 hour 45 minutes. Turn off oven, crack open door (use a wooden spoon if it won't stay open) and let meringue cool completely inside oven, about 2 hours.
4. Just before serving, whip cream until it holds stiff peaks. Spread on top of meringue. Top with berries and serve immediately.