The Vitality discount applies to the first £100 of Vitality food you buy per month (or £200 if you have a 2+ person Vitality membership). Any healthy food you buy after that will be charged at the standard price.
Here's an example. Let's say you have £20 remaining of your monthly £100, and you place an order buying £40 of healthy food. With your Gold 20% discount, you'll receive a £4 discount (20% of the remaining £20); not an £8 discount.