Apply the discount for tax included and excluded customers

We sell mostly to customers that pay VAT/Tax, but some customers are excluded from paying Tax. Our store shows prices as Exclusing VAT (until you get to checkout and it adds the VAT.). I've added the price for the discount in ex VAT (as per the rest of our products) but it's calculating it as ex VAT at checkout. Also, If I log into to an account that has VAT excluded, it shows the same price. We want it to include the tax when applying discount and change for VAT excluded or VAT included customers.