Hello,
As long as the coupon target one specific vendor, it will apply its discount only on the targeted vendor's products.
Afterwards, and depending how your products are organized, you can still use the HikaShop "category restriction" on the coupon/discount which can also be useful to target only products of specific vendors (as long as the "vendor root category" is configured and used)
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.