Preheat oven to 350 degrees. Thoroughly spray a large (18 x 12) sheet cake pan with nonstick baking spray.
Sift together the flour and salt. Set aside. In a separate bowl, stir together the buttermilk, vanilla, baking sodas and eggs. Add the vinegar and stir. Set aside.
In a separate small bowl, mix together the cocoa and red food coloring. Set aside. Cream together the shortening and sugar until fluffy. Alternately add the flour mixture and the buttermilk/egg mixture until all combined. Pour in the red mixture and beat until combined.
Pour the batter into the prepared cake pan. Use a large spatula or knife to even out the surface. Bake for 20 minutes. Remove from oven and let cool for 20 minutes.