vat_number_save.php

<?php
// Do NOT include the opening php tag shown above. Copy the code shown below.

add_action( 'woocommerce_checkout_update_order_meta', 'wpdesk_checkout_vat_number_update_order_meta' );
/**
* Save VAT Number in the order meta
*/
function wpdesk_checkout_vat_number_update_order_meta( $order_id ) {
    if ( ! empty( $_POST['vat_number'] ) ) {
        update_post_meta( $order_id, '_vat_number', sanitize_text_field( $_POST['vat_number'] ) );
    }
}