Yes, you absolutely use vegan butter to make frosting, but the texture will not be the same. Vegan butter (also known as margarine) is made out of oils, which are not as solid as butter, even when refrigerated. If you choose to use vegan butter to make this recipe, wait to add milk until you see what the consistency is like without it. If it needs to be thinned out a little bit, add a very small amount at a time.