Hello,
If you take a look at the link in my previous message, you will see that my custom field is a product custom field.
It's because Vendor can edit the products.
Now you want to talk about order custom field specifically.
But vendor can't edit orders except for the order status. That's a restriction since the very first version of HikaMarket and that's something which should change with HikaMarket 2.x but under very (very !) strict conditions.
Only the main vendor can edit the orders ; that's the current rule.
Afterwards, with the configuration of the HikaMarket ACL and the settings in the fields ; you can choose which fields are editable, when the user have actually the possibility to edit the element.
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.