I tried installing vendetta on my Gentoo system, but the installer gives me a checksum errror.
I've downloaded it with two different browsers. The website says it's 52k but it actually
downloads at ~142k.

vendetta-linux-installer.sh should be 152525 bytes.. I think the website means to say 152k (I just fixed that). Anyway, it sounds like your file is truncated or something. Very strange.

I just made sure it had the correct MIME type; maybe that helps? Be sure to clear your cache before trying again.