Here is my working config with apache 2.2.22

Address format: plex.hostname.com

I used hostname instead of localhost in the ProxyPass to be able to use Plex authentication for login.

<VirtualHost *:80>
ServerName plex.hostname.com

<Proxy *>
    Order deny,allow
    Allow from all
</Proxy>

ProxyRequests Off
ProxyPreserveHost On
ProxyPass / http://hostname.com:32400/
ProxyPassReverse / http://hostname.com:32400/

RewriteEngine on
RewriteCond %{REQUEST_URI} !^/web
RewriteCond %{HTTP:X-Plex-Device} ^$
RewriteRule ^/$ /web/$1 [R,L]

</VirtualHost>