Yes, you can use vegetable shortening in this recipe. Use up to 50% of vegetable shortening in place of vegan butter to make a vegan frosting that is more stable at room temperature, not to mention easier to work with. No, you can not use coconut milk to replace the butter. Coconut oil and coconut milk are too different from vegan butter and vegan shortening. However, you can use coconut milk to thin out the recipe a bit in place of the plant-based milk. That should give the frosting a subtle coconut flavor.