The commission rate can be set up for all vendors from the → Commissions settings page.
Yet, this option can be overridden at vendor and product level, so you can make sure that one or more vendors have a different commission rate or that specific products grant a different vendor commission rate.
Custom commission rate by vendor
To override the commission rate for one or more vendors, you can do that from the vendor’s profile.
Go to Vendors list, and click on the pencil icon next to the vendor you want to update.
Go to the Options tab and select Set a different commission percentage for this vendor. Then, add your custom rate here.
This option can be further overridden at product level (see below).
Custom commission rate by product
To make sure, instead, that a specific product grants a specific commission to the vendor assigned to it, you can simply go to the product settings page.
From the Product Data metabox, go to Commissions and add the custom percentage here.
Note: the Commissions tab will only be visible if this product has been already associated to one vendor.
These settings will override the general and vendor rate settings. In case if you set a custom rate for both the vendor and a specific product, the product rate will be applied, as more specific.