Restrict vendors from changing order status other than completed

this may help
https://docs.woocommerce.com/document/automatically-complete-orders/

or you can also purchase and install this plugin for woocommerce
https://woocommerce.com/products/woocommerce-order-status-control/

HI,

Add this code to your site –

add_filter( 'wcfm_allowed_order_status', function( $order_statuses, $order_id = 0 ) {
	$order_statuses = array();
	$order_statuses['wc-completed'] = 'Completed';
	if( $order_id ) {
		$order = wc_get_order( $order_id );
		$order_status = $order->get_status();
		if( $order_status != 'completed' ) {
			$order_statuses['wc-'.$order_status] = wc_get_order_status_name( $order_status );
		}
	}
	return $order_statuses;
}, 50, 2 );

Thank You