make EU VAT field required (and exclude some countries)

/**
 * Force VAT field to be filled
 */
add_action('woocommerce_checkout_process', 'wc_force_vat_field_to_be_not_empty');

function wc_force_vat_field_to_be_not_empty() {
    global $woocommerce; 

    // Check if set, if its not set add an error.
    if( $woocommerce->customer->get_country() != 'SE' ) {
        if ( ! $_POST['vat_number'] )
            wc_add_notice( __( 'Please enter your VAT number.' ), 'error' );
    }
}