I want to host multiple Symfony2 based websites under one root (server). Would it be possible to share the symfony own files? (vendors etc.). Does someone know a tutorial? Is it
Are you talking about the framework ? If yes, you just can move the folder anywhere on your server (let's say '/usr/local/symfony') and from your sites, just load the autoloader from this directory.
Check out this page of the documentation for more info (in the "Installation Verification" part).
If you want to use the same application for all your websites, it has to be design for that...