Hello,
Item/Product and order custom fields have some restrictions features ; you will find for example "products" and "categories" restrictions.
While HikaShop allows plugins to create new "field type", new "extra attributes" and new "display" blocks, there is no extension for the "restrictions" part.
But since you have a "categories" restrictions for the fields ; you can use the vendor categories in order to have your vendor restriction.
You can also use classical categories and mass action (or other) in order to add products into categories depending their product vendor id.
I think it would be the easier without the need of custom development (and modification in the core).
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.