One way to do this would be to use a Math equation in the Smarty template:
Code:
{math equation="price*((100+vatamount)/100)" price=$products[product].price vatamount=$products[product].vat format="%.2f" assign="incvat"}
the 'vatamount' needs to be the percentage of vat i.e 17.5
incvat variable would then include your vat inclusive price.
Please note that this is taken from our version of XCart which is 3.2.1, so this may differ in later versions.
But it gives you an idea.