I see!! So I changed the code like below. It seems to work
Thanks
<?php if ( $site = get_field( 'vendor_fb' ) ) : ?>
<?php
$site = str_replace(array('http://', 'https://'), '', $site);
$site = rtrim($site, '/');
$url = get_field( 'vendor_fb' );?>
<a class="vendor-link" href="<?php echo $url ?>" target="_blank"><?php echo $site ?></a>
<?php endif; ?>