why would you need a VPS for a simple forum?
Elaborating on the idea: Also for Web Hosting, which allows you to host other people, ect. (( cost alot more than shared hosting )), or stream live radio (( which also cost alot ))There's lots of shared hosting services out there that would gladly host a PHPBB3 forum for you free of charge. The services of a VPS are usually only needed for content with a complex configuration or special requirements such as a game server or custom service, regular websites like blogs and forums are well suited to shared hosting.
Put a '.htaccess' file with the following contents:
DirectoryIndex phpBB3/index.php
Or use page redirects from CPanel to redirect automatically to your forum path.