And while US has regretted for their crimes, China does not.
The US or the West regrets nothing done in Vietnam. In fact, Southern Vietnamese like Gambit flooded for Western refuge, and are living lavishly abroad, while Vietnamese earn less $ than Indians.
The biggest traitor in the Vietnam war was Vietnamese. The largest massacres of Vietnamese in history are the Americans. If you still love them after they killed 3 million of your kind.
The more Vietnamese you kill, the more they love you.
Also most of the US government and military officials were promoted because of their Vietnam experience, and US have Vietnam Veteran memorial wall to honor those who died in Vietnam. That seems really regretting to me, maybe the only thing they regret is that they did not win the war.