I thought they were just kidding, but maybe there is some real animosity there.
I think that when a neighboring country has been attempting to conquer your country for several hundred years only to get pushed back over and over again you probably start to have a chip on your shoulder about said people. I don't know the full history but yeah, the Vietnamese have a very fragile relationship with their northern neighbor. This probably explains a lot of why they always keep the Americans close and allow regular shows of US Naval might in the region.