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

add_action( 'woocommerce_admin_order_data_after_billing_address', 'wpdesk_vat_number_display_admin_order_meta', 10, 1 );
/**
 * Display VAT Number in order edit screen
 */
function wpdesk_vat_number_display_admin_order_meta( $order ) {
    echo '<p><strong>' . __( 'VAT Number', 'woocommerce' ) . ':</strong> ' . get_post_meta( $order->id, '_vat_number', true ) . '</p>';
}