Grease and line 2 baking sheets with nonstick baking paper. Whisk the egg whites in a thoroughly clean bowl until stiffly peaking. Gradually whisk in the sugar, a dessertspoonful at a time, whisking well between each addition, until the mixture is stiff and glossy. Drizzle the vinegar into the bowl and sift in the cornflour. Stir gently.
Place 20 spoonfuls of the mixture, each about the size of a golf ball, on the lined baking sheets and flatten into a neat dome shape with a palette knife. Push a deep cavity into each centre using the back of a teaspoon. Bake in a preheated oven, 140°C (325°F), Gas Mark 1, for about 1 hour until crisp to the touch. Leave to cool on the baking sheets.
Scoop the pulp from 3 of the passionfruit and press through a sieve into a bowl. Add the cream and yogurt and whisk with a hand-held electric whisk until softly peaking. Spoon into the meringue cases. Scoop the pulp from the remaining passionfruit and spoon over the tops. Decorate each pavlova with a physalis.