Actually he has already sold a huge part of his total Ethereum assets before. And the usual thing that will happen is the price will go down.
Everyone would say that if Vitalik Buterin is selling all his ETH, he knows about something serious that we don't know about.
A bear is coming? the project is failing? seriously, it's just all about taking his profits.
Yes profit will be taken, the price will go down and soon another rally will start. This is now the normal for cryptocurrency. Cryptocreency has gone beyond the stage of centralised system like Amazon. Sell of Ethereum can't crash the market but can only cause hodlers who are the whales to take profit and later buy more and that will waken the market again, that is the circle of cryptocurrency.