Vitalik is just one of the members of the Ethereum development team. Maybe he is the most important person, but he doesn't mean all. In other words, if Ethereum left Vitalik still alive and will not die.
Now there is one of the best case is Bitcoin. After the creation of Bitcoin, Nakamoto Satoshi has disappeared, but Bitcoin has not died because of the departure of Nakamoto Satoshi. On the contrary, Bitcoin is now alive.