A calculation is on a per item basis and can only include values from the cart columns. It will not be possible to include other charges from the cart in a calculated column. You could store the total vat for each item with a new column in the cart, then you could setup a charge rule that will be based on the subtotal of this vat column in your cart. Please explain a little about how you would like this working in the end and we can give you some pointers on how to get it that way.