Vitalik and the devs could certainly be approached by a government and be coerced into submitting malicious code that would harm the community. This as unlikely as it may be isn't realistically going to happen. Why? Vitalik doesn't decide what code runs on the network, the nodes and miners decide what forks happen and will be decided as the official chain. Who in their right mind is going to choose code that wouldn't work in their best interest? Long before it would get a chance to be implemented someone would catch on and many would not adopt the code.
What if the government gave him the order to create some complicated upgrade which had the "unintended consequence" (properly obfuscated in the code) of doing what the government wanted? Few, if any, would detect it.
Good question. I believe the code being open source would reduce the probability of this happening. If this could be pulled off then it could be done with ANY crypto not just ETH. BTC would be at risk also. I believe that if the government wanted to alter the game they would have the money and resources to 51% attack instead.
This could happen in the ETC land as well. I'm not leaving Vitalik, the brain behind Ethereum. To leave the road that he's travelling on is foolish. It takes wit, dedication and passion to develop such complex platform as Ethereum.