Hi,
There is no option for that in shipping methods. It would require a custom shipping method to be built to check on the VAT of the shipping address.
It would actually be simpler for you to not require the VAT number field and instead create a small hikashop plugin implementing the onBeforeAddressCreate and onBeforeAddressUpdate triggers and do the check on emptiness on the VAT number field based on the country field. That should be the best solution here compared to all the other ones we talked about although it requires to be a developer as there are a few lines of code to write, but for a normal PHP developer, it should not be a problem.