Vietnam became unified with both north and south as one country, in 1802.
{...
Unified era (1802–1858)
After
Nguyễn Ánh established the Nguyễn dynasty in 1802, he tolerated Catholicism and employed some Europeans in his court as advisors. His successors were more conservative Confucians and resisted Westernization. The next Nguyễn emperors,
Minh Mạng,
Thiệu Trị, and
Tự Đức brutally suppressed Catholicism and pursued a 'closed door' policy, perceiving the Westerners as a threat, following events such as the
Lê Văn Khôi revolt when a French missionary, Fr.
Joseph Marchand, was accused of encouraging local Catholics to revolt in an attempt to install a Catholic emperor. Catholics, both Vietnamese and foreign-born, were persecuted in retaliation. Trade with the West slowed during this period. There were frequent uprisings against the Nguyễns, with hundreds of such events being recorded in the annals. These acts were soon being used as excuses for France to invade Vietnam.
...}
en.wikipedia.org
The French then invaded in 1858.
{...
The
French colonial empire was heavily involved in Vietnam in the 19th century; often French intervention was undertaken in order to protect the work of the
Paris Foreign Missions Society in the country. In response to many incidents in which Catholic missionaries were persecuted, harassed and in some executed, and also to expand French influence in Asia,
Napoleon III of France ordered
Rigault de Genouilly with 14 French gunships to attack the port of
Đà Nẵng (Tourane) in 1858. The attack caused significant damage, yet failed to gain any foothold, in the process being afflicted by the humidity and tropical diseases. De Genouilly decided to sail south and captured the poorly defended city of
Gia Định (present-day
Ho Chi Minh City). From 1859 to 1867, French troops expanded their control over all six provinces on the Mekong delta and formed a colony known as
Cochinchina.
...}
So Vietnam had been unified since 1802 until it was divided for election purposed in 1954.
But I conceded that I can't find an election for Ho Chi MInh.
But you are wrong about what the Constitution does and what the UN charter being ratified by congress means.
The Constitution does not say what US law is, but instead just defines what federal jurisdiction and authority encompasses.
And definitely congress does have jurisdiction to legislate restrictions on war if it wants.
And when Congress ratifies any treaty, it becomes US law.
So when Congress ratified the UN Charter, it then became US law, and made any US aggressive war illegal, by US law.
The UN has nothing to do with that. It is just that if the UN authorizes the use of force, as it did in Korea, than the restriction against US aggressive use of force no longer is a restriction.