My understanding (I'm not an accountant) is that what matters is what's on the receipt/invoice. So if you use VIP points to get $20 off, the price is discounted by $20 for tax purposes. This is why it makes sense to use VIP points for personal sets. Same with Kohls Cash and similar discounts. Earn them while buying Lego to sell, use them for personal purchases.
Credit card rewards I have never factored when computing my cost. I mostly go for travel rewards, which are difficult to value anyway. But even for cash back, the redemption isn't tied to a particular line item on the bill. So unless you strictly separate your business card from personal use, I wouldn't know how to do it.