Hook checkout / cart based on order fields

For a new project I am thinking on use basic_cart again. But this time I have a special need and maybe someone has an idea on how this could be done using basic_cart:
I need to add VAT for customers from Germany.
If a user is from EU (but not Germany) I need to remove VAT if the user enters a valid VAT ID (I use the vat_number module). If the user add a country within EU and vat_number I want to update the cart - maybe using an "update cart" button.
Anyone an idea on how this could be done using a hook or a custom module?