WC Vendors - Forcing all new registrations to be Vendors

functions.php

/* Forces all new registrations to your site on the My Account page to be a Vendor, rather than the Customer role */
add_filter( 'woocommerce_new_customer_data', 'custom_woocommerce_new_customer_data' );
function custom_woocommerce_new_customer_data( $data ){
    $data['role'] = 'vendor'; // the new default role
    return $data;
}