I found somehing that -I think- accomplishes what I want.. Well.. Almost:
http://www.makina-corpus.org/blog/insta ... ot-php-fpmThis one is more for just Drupal, I just want something in general, and this one doesn't use vhosts. So it's not exactly what I want.
It's kinda strange there aren't many guides about this online. I thought that especially professional web hosters/resellers (with shared web hosting) would use things like this all the time to protect their hosting environments. After all, they don't want people being able to upload some kind of php shell and simply read out server files, or exploit it in any other way. Plus they add new vhosts all the time when a new customer registers, automatically even. It can't be *that* hard :/