Vitalik is the face of Ethereum. When the name Ethereum is said, many people think of Vitalik as the main guy. It's like Facebook, we all think of Mark Zuckerburg as the face of facebook. Thus, if something happens to him, the price of Ethereum would definitely tank.

However, Ethereum would not totally die. There is a team behind Ethereum, and the team would probably work harder. There would also be a new "Vitalik", someone who would be representing Ethereum, and people would then treat him as the face of Ethereum.