Preheat oven to 300 degrees. Lightly grease a 10-inch tube pan. Place cooking rack in center of oven. Combine vanilla, lemon juice and Amaretto in a small bowl. Set aside. Beat egg whites in a large bowl until frothy. Add cream of tartar and salt. Continue beating until soft peaks form. Add sugar, ⅓ cup at a time. Continue beating until stiff, but not dry. Fold in vanilla mixture. Sift flour over egg whites and fold gently. Spoon batter into prepared pan. Bake 1 hour, or until cake is golden brown and springy to the touch. Cool completely while inverted. Remove from pan.