Mistake or not, it was necessary or the Americans would have established a base in Vietnam. Look at South Korea and Japan. The problem with these Viets are the wars that broke out between China and Vietnam in ancient times to the Cold War era to the fighting in the 70's and 80's regarding SCS. Can't really blame them for their hostilities and yes they certainly are ungrateful/arrogant because they actually think they could have beaten US without China's help. They are so blinded by that hatred they forgot they once turned into a French and Japanese colony and how many of their own people died by these powers. They rather beg the devil that almost destroyed them during the Vietnam war.
Breaking them into 2 parts is no use because the people in South Vietnam either fled to US or were captured by the Vietcongs and turned them into communists after the war.