If you referring to the placeholder attribute that shows a value inside an input field until someone enters something in it, the EU VAT Assistant already adds one. It’s automatically set to the value of the field title.
If you’re looking to change that, you can use the same standard filter used by the plugin, i.e. woocommerce_checkout_fields. Example:
add_filter('woocommerce_checkout_fields', function($fields) {
$fields['billing']['vat_number']['placeholder'] = 'SOME PLACEHOLDER TEXT';
return $fields;
}, 50);
For information on how to add or modify the checkout fields, please refer to the official WooCommerce documentation: https://woocommerce.com/document/tutorial-customising-checkout-fields-using-actions-and-filters/#section-2
