If you want to, your vendors can create and manage coupon codes for their own products only. You can enable the Create coupons to be used on his/her products option from the → Vendors’ permissions.
Now, the vendor will be able to see a Coupons menu in his/her dashboard and create coupons. These coupons will be automatically enabled only for the products of the vendor who generated the coupon. Yet, the vendor can further filter the products it can be used on and use all the default coupon settings.