The setting is dystopia because there is a need for the government to control its people. The government feels that if people would suddenly decide that they do not need the government to protect them, they would do what they want. In the process, people may become successful and overthrow the need for a government. In the case of Vendetta, the government used Vendetta as a tool so that people will become fearful for their lives.
This means that they have to rely on the government in order to feel safe. If you would think about it, this also applies to the life that we are living right now. There are things that we fear and we rely on the government to tell us what to do next.
Explanation: The setting depicted in the V for Vendetta trailer is a dystopia because the government tries to use fear as a tool to rule its people. If the people fear an external threat (in this case the external threat is V, according to the government), they will feel as though they need the government to protect them.