What mistakes did Nixon make in Vietnam?
This depends greatly on your perspective and on what you would call a mistake.
If you look only at Nixon's own goal, it is possible to argue that he made no mistakes. Nixon's main goal was "peace with honor." He wanted to get the US out without looking like it was simply surrendering. He managed this through his process of Vietnamization and his push for peace talks. Obviously, Vietnamization did not work to create a stable and strong South Vietnam, but it is hard to argue that this was Nixon's fault rather than the fault of the South Vietnamese and perhaps of Nixon's predecessors in office.
You could argue that Nixon's decision to widen the war by invading Cambodia was a mistake. You can argue that this was a moral mistake because it hurt a country that was not officially a part of the war. So, if you are looking at things from a moral standpoint, you can argue that this was a mistake. It might also have been a mistake in that it helped turn the American public even further against the war. However, it is hard to argue that the invasion of Cambodia helped cause the US to lose the war.
So, it is very much a matter of opinion as to which of Nixon's actions were mistakes.