Add the vendor role to an instructor when they are approved.

functions.php

<?php 

/**
* Required Tutor LMS v.1.6.0
*/

add_action('tutor_new_instructor_after', 'wcv_add_vendor_role');
/**
 * @param $instructor_id
 *
 * Add thhe vendor role to the user when they are approved to be an instructor. 
 */

if ( ! function_exists('wcv_add_vendor_role')){
    function wcv_add_vendor_role($instructor_id){
        $instructor = new \WP_User($instructor_id);
        $instructor->add_role('vendor');
    }
}