Set I am a vendor by default

functions.php

remove_action( 'woocommerce_register_form', 'dokan_seller_reg_form_fields' );

add_action( 'woocommerce_register_form', 'dokan_custom_reg_vendor_selected', 12 );
function dokan_custom_reg_vendor_selected() {
    $postdata = wc_clean( $_POST ); // WPCS: CSRF ok, input var ok.
    $role = isset( $postdata['role'] ) ? $postdata['role'] : 'seller';
    $role_style = ( $role == 'customer' ) ? 'display:none' : '';
    dokan_get_template_part( 'global/seller-registration-form', '', array(
        'postdata' => $postdata,
        'role' => $role,
        'role_style' => $role_style
    ) );
}