I don’t think a simple code addition would be able do this. You would need automation app. There are a few apps available in the app store. I would suggest you to try some of the apps and talk to the app support. Hopefully they would be able to guide you.
This will need to make a lot of changes to the code, if you have experience with liquid and JS it will help you a lot.
You need to create if/else customer has a VIP tagged, there will be a hidden display product and at the same time make the add to cart event implicitly with JS.
Ideally, you should hire a professional, it will do this quickly in more than 1 hour.
Hope it helps!
Shopify Ajax API to check if the product is in the cart already, and if not then add it: https://shopify.dev/api/ajax
If you don’t have a developer on your team and want this handled for you feel free to contact my team at speedboostr.com/contact, my team does customizations like this all the time.
If you do take a crack at it yourself, make sure you backup your theme and work on the backup theme (you should always do this when making theme changes, to ensure you have a backup in case something goes wrong).