Hello,
When the product do not have a vendor set-up, the vendor will be your store (the main vendor).
Also, when a product do not have a vendor, you can use feature such as the "dynamic vendor attribution" like the "vendor selector" or custom plugins.
So changing that behavior in HikaMarket would result to lot of side effects and problems.
But I will create a issue ticket regarding the fact that the error message returned by HikaMarket when it refuses that the product is added into the cart, is not catch by the HikaShop core.
Kind 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.