wc_vendors_custom_percent_commission.php

<?php

add_filter('wcv_commission_rate', 'm_commission_update', 20, 5);

/**
 * @param $commission
 * @param $product_id
 * @param $product_price
 * @param $order
 * @param $qty
 *
 * @return float
 */
function m_commission_update($commission, $product_id, $product_price, $order, $qty) {

	$author = WCV_Vendors::get_vendor_from_product( $product_id );

	if ( WCV_Vendors::is_vendor( $author ) ) {

		// the commission you want to add - switch to whatever logic fits
		$card_commission_rate = 1;

		// Maximum commission ( optional )
		$max_commission = 299;

		// add the card commission
		$m_commission = $product_price * ( intval( $card_commission_rate ) / 100 );
		$m_commission = round( $m_commission, 2 );

    // Comment this out if don't want a cap
		if ( $m_commission >= $max_commission ) {
			$m_commission = $max_commission;
		}

		// remove the commission from the payment that goes to the vendor
		$commission -= $m_commission;

	}

	return $commission;

}