I actually found the answer to your questions in their T&C, copied belowNo, I didn't get an answer, except that I tried it out so will have to wait for my cashback to arrive mid-April and then try to work it out! I suspect you are correct that I won't be getting the Vitality Money portion.
How to earn rewards on your HealthyFood purchases
To earn rewards from the Vitality Health
If you do not present your HealthyFood partner loyalty card, you will not earn any HealthyFood rewards from either the Vitality Health or Vitality Money programmes.
If you do not pay using your qualifying Discovery Bank credit card, you will only earn rewards on your HealthyFood spend from your Vitality Health programme and not the boosted HealthyFood reward from your Vitality Money programme