Are you using a dedicated IP address for that site (i.e., not name-based hosting)? If you're using name-based hosting, the configuration of the domain is bypassed when you try to access the site by its IP address. That's because the VirtualHost settings (and therefore the associated vhost.conf settings) are based on the hostname, not the IP address.
If you want to change the server-wide settings for directory listings (not recommended...), you can edit /usr/local/psa/apache/conf/httpd.conf and change
CODE
<Directory />
Options FollowSymLinks
AllowOverride None
</Directory>
...to...
CODE
<Directory />
Options Indexes FollowSymLinks
AllowOverride None
</Directory>
Then, the directory listing will be shown for all domains and IP addresses in the absence of index.html, etc.