Hi Nicolas,
In another topic I found part of the solution and after some testing i used the following php code and put it at the bottom of the file plugins/hikashop/attachinvoice/attachinvoice/invoice.php just before the </page> tag.
<?php if (!empty($order->billing_address->address_vat) && $order->billing_address->address_country !== 'Netherlands'){
echo 'BTW verlegd naar: '.$order->billing_address->address_vat;
}
?>
At first I thought that I only had to check for address_vat but when a customer from the Netherlands is filling in this field then he also got that text line. So I had to do a double check see code above.
This works for me..
Have a nice weekend.