Hi,
The VAT is product related.
So for vendors who does not have any VAT ; they should select the corresponding tax category for their products so the product won't have any tax applies.
And the same thing for vendors with VAT ; they should select the right tax category for their products.
So, you need to let them access to the product tax category in the market ACL.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.