In a bowl, combine the sugar and vanilla. Set aside.
In another bowl, beat the egg whites until they form soft peaks. Gradually add the vanilla sugar and beat until the meringue forms stiff peaks. With a spatula, gently fold in the yogurt. Pour into 250 ml (1 cup)-capacity dessert cups or tall glasses. Cover with plastic wrap and refrigerate for at least 1 hour.
To serve, top with pieces of Stewed Plums (see recipe) and some of their syrup on te mousse. Yogurt mousse is best the same day.