I really don't see a point to adding it. If someone wants the benefit of vitamin D, they can get from some sun or they can take a D3 supplement. Adding it to a product that won't even stay on your skin for more than 5 minutes is pointless and an expensive waste of a much needed vitamin. Let's not forget- chemical changes are happening in that soap to who knows if the vitamin will be inactivated.I add antioxidant Vitamin E (1,000 IU) ppo to every batch. I feel it adds conditioning as well as longer shelf life. When it comes to Vitamin D, I think the sky's the limit, not knowing what purpose it serves in the soap.