Hello,
Currently, the only coupons or discounts that work properly with vendor commissions calculations are the Vendor coupons (pro dashboard > coupons) built into WC Vendors.
WooCommerce coupons only calculate properly if used for the admin’s products if you have products listed as the admin.
Hopefully in the future we’ll be able to add some more coupon options, but that will require a re-write of how commissions is calculated. Although this work (re-write) is planned, we do not have a date set for the work to be completed.