Holy crap, guys. I discovered last night that just about every single epub in the Vault was corrupt. No wonder it's doing so badly in the format poll!
I have corrected, validated, and spot-checked all the epubs and reuploaded them - so if you've been desperate to use one but have been getting weird error messages when trying to read it, redownload!
On a related note, some of the formatting in the mobi files has been cleaned up, so if you're a mobi user, you might want to redownload too.