YITH WooCommerce Dynamic Pricing and Discounts allows you to create any sort of discount or special offer for your shop.
The integration with YITH Multi Vendor gives you the possibility to create custom discount rules for all the products associated with one vendor or to provide a discount for all the products of the shop except for those linked to a specific vendor.
Just make sure you have installed and activated the premium version of both plugins.
Then, whenever you create a price rule, you will be able to restrict it to products by one or more vendors, whether it is a Price rule…
… or a Cart discount rule:
For more details about YITH Dynamic Pricing and Discounts, please, refer to the plugin official documentation.