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.
Its his money and selling is not a bad thing, beside if a project is a good one it can stand alone even if the founder sell his holdings because there are so many investors that are willing to buy at a cheaper price and if Vitalik sold some of his holdings again, then the price will just correct for a little but after that it will rise again as long as they don’t abandon ETH.