In a small saucepan sprinkle the gelatin over 1/4 cup cold water, let it soften for 1 minute, and heat the mixture over low heat, stirring, until the gelatin is dissolved. In a blender blend together the mango purée, the sugar, and the vanilla, add the gelatin mixture, and blend the mixture well. Transfer the mixture to a bowl and stir in the yogurt. In a chilled bowl beat the cream until it holds stiff peaks, fold it into the mango mixture gently but thoroughly, and divide the mousse among 6 dessert glasses. Chill the mousses for at least 4 hours or overnight. Garnish the mousses with the mango slices.