When a major change happen in ethereum like Softfork or Hardfork, all nodes may not agree with this. So is this change possible ? Vitalik is creator of Ethereum. But ethereum is totally decentralized, so now Ethereum is a large network. Now Have vitalik any control over Ethereum network ?
There are bascally two kinds of forks: hard and soft.
Soft fork is a change in block verification rules, that makes verification more strict: all blocks that are valid according to the new rules are also valid according to the old rules, but not vice versa. After the fork, some nodes may not be upgraded, either by mistake or for reason, but even though, if majority of miners will agree with fork, then blockchain branch based on the new rules will quickly superseed old-rules branch, and even non-upgraded nodes will switch to it, because all the blocks in new-rules branch are valid for them. That's why soft forks are considered more safe: successful soft fork (i.e. supported by the majority of miners) does not actually fork the blockchain.
In contrast, hard fork introduces verification rules, that allow blocks considered invalid according to the old rules. Non-upgraded nodes will never accept such blocks, so even successful hard fork carry a risk of permanent network split, in case some nodes and miners will not upgrade. For example, miners and nodes,that ignored "The DAO hard fork" formed separate blockchain known as Ethereum Classic.
Vitalik controls Ethereum using his soft power to convince miners and node owners to upgrade their software.