Good question and at it's heart one that has already interested me. How does conjuration work in a system.
Is the conjured object (Milk in this case) created spontaneously or is it summoned from elsewhere (elemental plane of milk, random containers or lactating creatures around the world) or is it borrowed milk from the past or the future.
If its spontaneously created I would probably say it was vegan, unless it now counts and the lactomancers milk (and said conjurer is not a plant).
If it is summoned/borrowed milk, than it is most likely not vegan, and the conjurer may be guilty of thievery.