Wrong.
Vietnam was unified before the French.
{...
Nguyễn dynasty (1802–1945)
...}
en.wikipedia.org
The UN is not meaningless because we not only created the UN, but congress ratified its charter into US law.
So only the UN could have intervened in Vietnam, not the US.
We were in violation of US and international law.
Everyone involved was a criminal.
Diem was not the legal leader of anything. Bau Dai was to be the temporary leader in the south, until elections.
Diem prevented free elections.
He was a criminal, and eventually even the US had to turn on him and have him killed.
{...
As the Buddhist crisis deepened in July 1963, non-communist Vietnamese nationalists and the military began preparations for a coup.
Bùi Diễm, later South Vietnam's Ambassador to the United States, reported in his memoirs that General
Lê Văn Kim requested his aid in learning what the United States might do about Diệm's government.
[141] Diễm had contacts in both the embassy and with the high-profile American journalists then in South Vietnam,
David Halberstam (
New York Times),
Neil Sheehan (United Press International), and
Malcolm Browne (Associated Press).
[142]

Ngô Đình Diệm after being shot and killed in the 1963 coup
The coup d'état was designed by a military revolutionary council including
ARVN generals led by General
Dương Văn Minh. Lieutenant Colonel
Lucien Conein, a CIA officer, had become a liaison between the US Embassy and the generals, who were led by Trần Văn Đôn. They met each other for the first time on October 2, 1963 at Tân Sơn Nhất airport. Three days later, Conein met with General Dương Văn Minh to discuss the coup and the stance of the US towards it.
[143] Conein then delivered the White House's message of American non-intervention, which was reiterated by
Henry Cabot Lodge Jr., the US ambassador, who gave secret assurances to the generals that the United States would not interfere.
[144]
The coup was chiefly planned by the Vietnamese generals.
[143] Unlike the coup in 1960, the plotters of the 1963 coup knew how to gain broad support from other ARVN officer corps. They obtained the support of Generals
Tôn Thất Định, General
Đỗ Cao Trí, General
Nguyễn Khánh, the III, II Corps and I Corps commanders. Only General Huỳnh Văn Cao of IV Corps remained loyal to Diệm.
[145]
On November 1, 1963, Conein donned his military uniform and stuffed three million Vietnamese
piastres into a bag to be given to General
Minh. Conein then called the CIA station and gave a signal indicating that the planned coup against President Diem was about to start.
[146] Minh and his co-conspirators swiftly overthrew the government. With only the palace guard remaining to defend Diệm and his younger brother Nhu, the generals called the palace offering Diệm exile if he surrendered. That evening, however, Diệm and his entourage escaped via an underground passage to Cha Tam Catholic Church in
Cholon, where they were captured the following morning. On November 2, 1963, the brothers were assassinated together in the back of an
M113 armored personnel carrier with a
bayonet and
revolver by Captain
Nguyễn Văn Nhung, under orders from Minh given while en route to the Vietnamese Joint General Staff headquarters.
[147] Diệm was buried in an
unmarked grave in a cemetery next to the house of the US Ambassador.
...}
en.wikipedia.org
You can not possibly defend Diem and then defend the US having Diem killed as well.
(By the way, the references to Minh refer to General Dương Văn Minh , not Ho Chi Minh, in the quote.)