Hi there,
ViArt did try to address this by adding the ability to add Multi-Tax rates per country.
Please check the section "Ability to add Multiple Tax rates per Country" on this page for more information:
http://www.viart.com/viart_shop_v.3.6_released.html
If this does not address your requirement, please do send us a support request with more information and include any documentation if possible.
Regards,
Ravs
ViArt Team