Preheat oven to 350. Grease and flour two 9 inch round baking pans. In a large mixer bowl, beat butter, sugar and vanilla until creamy. Add eggs and food color; blend. Stir together flour, cocoa and salt; add alternately with buttermilk to butter mixture. Stir baking soda into vinegar; fold into batter (do not beat) Pour batter into prepared pans. Bake 30 to 35 minutes. Test for doneness. Cool at least 15 minutes; remove from pans to wire racks. Cool completely before frosting.
Frosting: In a large bowl, beat butter, 1 cup powdered sugar, vanilla and salt. Add remaining powdered sugar alternately with milk, beating to spreading consistency. About 2 1/2 cups of frosting.