Candelle, I´m sorry, I don´t have a formula for swapping butter for oil, most of my cakes are just a bit of this and a bit of that until the consistency is about right. My favourite vegan cake in different variations is using cups (any size, not necessarily the American ones): 3 cups flour, 3 cups mango (or pumpkin, or sweet potato, or apple, or anything) purée, 1 cup sugar, 1 cup oil, 1 tsp baking powder, pinch of salt, spices of any kind (cardamom, cinammon, cocoa powder, etc. Not very helpful, I know. So, roughly 3 of flour, to 1 of sugar and 1 of oil plus something else moist, almond milk would work too.