If by the document you mean the original Ethereum yellow paper that first formalized it, the main author was actually Gavin Wood, not Vitalik :)

Both were crucial in laying down the technical and theoretical foundation of Ethereum.

Gavin is working on other things now. Vitalik is not the thought-leader he was in the early days (which is a good thing as he’s not a SPoF and decision making is more decentralized)