To allow each vendor to manage refunds in their orders, it is essential to activate the “Order refund management” option.
Refunds will be applied following the classic WooCommerce system: you need to go to the order detail page and set the amount of the refund for the single product of the order total.
When vendors add a refund in an order, this also visible to the administrators of the shop, with the related recalculation of the commissions.