I read some answers below. I only want to share about my experience in the first time traveling to Cambodia.
In Vietnam, some friends of mine told me that Cambodian do not like Vietnam, so When I talk to them it is better to me If I say I am Chinese.
I traveled to Banlung City from Gia Lai province with my motorbike in a adventure trip. When I met locals, they were very nice, friendly.
I had a chance to talk to some of them, and I said I was Vietnamese. They did not change their attitude, I appreciate that. A guy I met in Balung, he shared his experience to me when traveling to Vietnam as well.
I knew a litte bit about the history amid two nations, but it is the past, now the two countries are closer together.
By the way, the Khmer girls are so beautiful.