A hard fork is essentially a large coordinated migration from one blockchain to a new blockchain, generally with a shared history.
Vitalik does not have the sole power to go against the stakeholders of the network and make massive changes to Ethereum. The power that Vitalik has is one of leadership. If he attempts to coordinate a mass migration from one blockchain (old Eth) to another (new Eth), Vitalik actually needs to convince everyone that this is a good idea. He can't force it to happen if people disagree that it's the right thing to do. If Vitalik launches a hardfork and 80% of the network upgrades, that definitionally means that Vitalik had the support of 80% of the network, otherwise they wouldn't have followed the upgrade.
Blockchains are about consent. If you don't like what's happening on a blockchain, make a change. A single person making a change in isolation isn't interesting, but a large group of people making a change together IS interesting, and can be successful even if that group is just a minority. You end up with two different networks, each capable of thriving on their own.
Miners depend on the users of a blockchain for revenue. If a blockchain has no users, there will be no fees, and no revenue. The reverse is not true. If the users determine that they are better off selecting a different set of miners to build and preserve consensus, they can fork the network in a way that changes who is able to mine effectively, but does not otherwise impact daily use of the blockchain.