While I haven't personally tested this exact substitution, you should be able to replace the flour, cornmeal, baking soda, baking powder, salt, and sugar with two boxes of Vegetarian Jiffy Mix. Since the mix already contains shortening, you might not need to add extra butter. Instead, start by combining the mix with about 1 1/4 cup of vegan buttermilk. If the mixture seems too dry, add a small amount of melted vegan butter or oil. Please note, as this modification hasn't been tested, I can't guarantee the results. Be sure to let me know how it turns out if you decide to give it a try!