If you change the document root for that vhost, you’ll get /forum/index.php as /index.php, but you will no longer be able to see /phpmyadmin/.

It’s usually easier to put each application in it’s own vhost and point a bunch of names at your webserver. That will give you phpmyadmin.yourdoamin.com as one name, and yourdomain.com as the one that serves the forum.

Then you just need one virtualhost in your apache config and one A record for each name.

It also means you can use different authentication options on your phpmyadmin without it breaking other stuff by accident.