Preheat the oven to 120°C. Mix the egg whites and the first third of the caster sugar in a mixer on high speed until soft peaks form. Add the second third of sugar and mix for 5 minutes, then turn the mixer down to slow. Combine the remaining sugar with the cornflour and raspberry powder and add to the mixer with the red wine vinegar, beating until just combined. Spoon the mixture onto a tray lined with baking paper and spread out to about 23cm diameter. Cook in the oven for 55 minutes. Set aside the pavlova to cool, then remove from the baking paper to a serving dish (or store until needed). Top with whipped cream and fresh raspberries and serve.