Place the cranberries and water in a small saucepan. Cook for three or four minutes, until the skins pop. Cool.
Turn the cranberries, sugar, egg whites and salt into the top part of a double boiler over boiling water. Beat for seven or eight minutes, or until the frosting thickens and holds its shape. Remove from the heat.
Add the vanilla and continue to beat until the frosting is stiff enough to spread.