He can prove that CSW is a fraud though, and that's exactly what Vitalik is saying.

So he doesn't need to prove a negative per say, he merely needs to prove that Craig is a fraud as he's been saying for quite a while now.

And to be specific, that fraud is that CSW is claiming to be Satoshi Nakamoto.

Vitalik can bring along all the notes which Greg Maxwell and others have been producing, and see if it stands up in court. Against an apparent empty handed CSW and friends.