Add the below code on your functions.php file of your current active theme to add the main order id in the vendor dashboard along with the suborder id –

add_filter('mvx_datatable_order_list_table_headers', 'test_callback', 10, 2);
function test_callback($orders_list_table_headers, $current_user_id) {
    $orders_list_table_headers['main_order_id'] = array('label' => __( 'Main Order', 'multivendorx' ));
    return $orders_list_table_headers;
}
add_filter('mvx_datatable_order_list_row', 'test_function', 10, 2);
function test_function($vendor_rows, $order) {
    $vendor_rows['main_order_id'] = wp_get_post_parent_id( $order->get_id() );
    return $vendor_rows;
}