My first suggestion would be to do one thing at a time. I'll give you the steps doing two things at a time, but it might confuse you.
The first thing is to update your database structure... add the necessary information to your database to populate the version menu most likely in another related table. Also make sure there is a field in the products table that identifies 1=vat or 0=non-vat.
Then update your cart structure... go to your cart and add two columns: "Version" and "Taxable"
Then go to the calculations tab and add a calculation: AmountTaxed = [Taxable] * [TotalPrice]
Then you are able to work on the add to cart page... you write a query to return the version values the product displayed based on the database structure used to hold that data.
Then you drag and drop a list into your add to cart form and populate it from the recordset using the Dynamic button in the properties inspector.
Then you update the add to cart server behavior specifying the version from the list and the Taxable value from the field you added to your products table.