Preheat the oven to 275 degrees F. Line a baking sheet with a large sheet of parchment.
In a food processor, pulse the sugar to make it fine (or use extra-fine sugar) and set aside. In a medium glass bowl, whip the egg whites with an electric mixer on low speed until foamy, not quite making peaks. Add half of the sugar and whip until stiff peaks form. In a small bowl, sift the remaining sugar with the cornstarch. Add to the egg whites along with the vanilla and vinegar and blend until incorporated.
Spoon the mixture into 4 piles on the prepared baking sheet. Use a spoon to create a well in each pile. Bake for 40 minutes, and then turn off the oven and leave the pavlovas in the oven untouched for another 20 minutes. Remove from the oven and allow to cool.
Meanwhile, whip the cream until it makes soft peaks. Top the cooled pavlovas with the whipped cream and strawberries. Make a quick sauce by thinning the jam with the hot water and drizzle over the pavlovas. Serve.