Well, to get around the problem you're suggesting, all they would need to do is to use the "generalized key" that is out there for the whole public, run it through a key generator scheme like the Diffie-Hellman key exchange protocol and you've got a brand new key that uses a public and private key to be secure. I didn't read the article, so I've no idea how Vanish addresses this problem, but it is a very easy one to solve. Anyone in cryptography knows about the Diffie-Hellman algorithm and how well it works against Eve, however if Oscar were involved, that's another story. (Eve just listens, Oscar intercepts and modifies the packets).