guro wrote: ... where i put this {address_vat} tag ...
It's not a site template or CSS thing, but as Nicolas indicated, it requires the edition of two views:
In the HikaShop backend, go to -> Display -> Views (or whatever they are in your language).
In the window that opens, use the filters in the upper right to find the files named "address_template". You'll need to find it twice: once each for your frontend template ("gk_storefront") and your backend template ("isis", I guess).
For each of the two, open it by clicking on the name "address_template", then you'll see the present tags, and you'll have an idea where to add the new one.
Save, and it should be done and working.