In top of double boiler, combine egg whites, salt and orange juice. Beat mixture until egg whites hold their shape in soft peaks. Continue beating while adding agave in a fine stream. Cook over boiling water, beating constantly for 5 minutes until mixture forms stiff peaks when beater is raised. Remove from heat. Add vanilla and beat frosting 2 minutes longer until frosting is of spreading consistency. Makes 1 cup, enough to frost 2 8-inch cake layers.