Hi,
That's right, the best will be to perform a MySQL request directly.
You can use the table "hikashop_vm_prod" which make the link between the "virtuemart_product" and the "hikashop_product".
So you can use an "UPDATE" query to update the "hikashop_product" table from data coming from "vm_product_...." with a inner join on "hikashop_vm_prod".
Something like:
UPDATE #__hikashop_product AS hkp
INNER JOIN #__hikashop_vm_prod AS hkvm ON hkp.product_id = hkvm.hk_id
INNER JOIN #__vm_product_type_1 AS vmp ON vmp.product_id = hkvm.vm_id
SET
hkp.hk_customfieldkey = vmp.vm_customfieldkey
(where you will replace the two fields in the "SET").
Regards,