Take extra care filling cupcake liners with meringue. It is important that none of the meringue spills over the liner onto the muffin tin because it will make removing the baked cupcakes difficult.
Preheat oven to 225 degrees. Line every other cup of two nonstick 12-cup muffin tins with cupcake liners, and set aside. Combine the egg whites, salt, vinegar, and vanilla in the bowl of an electric mixer fitted with the whisk attachment. Beat on medium-high speed until frothy. Add the sugar, 1 tablespoon at a time, beating a full 2 minutes after each addition, making sure the sugar has completely dissolved. Use a rubber spatula to scrape down sides of bowl after each addition of sugar so that undissolved sugar does not build up. Egg whites should form glossy firm peaks.
Using a rubber spatula, spoon meringue equally into 12 paper cups so meringue is about 2 inches above rims. Clean up edges so no meringue is touching muffin tins. Do not smooth meringue or worry about peaks and odd shapes. Place in the oven, and bake until very light brown and completely dry on the outside but still soft in the middle, about 2 1/2 hours. If using one oven, rotate tins after 1 1/2 hours of baking. Remove from oven, remove cupcakes from tins, and peel off cupcake liners. Let cool.
Combine stewed rhubarb with raspberries. Combine creme fraiche and heavy cream; whip to soft peaks. Using a serrated knife, gently slice tops off cupcakes; do not worry if they crack. Spoon whipped creme fraiche into the center of each cupcake, and cover with stewed fruit. Cover with top of cupcake.