Edit: this was an April Fool's post. The real answer (which I am unable to develop to a full /r/AskHistorians level) is that the French-speaking Vietnamese, since the Vietnam war(s), now live in the 13th arrondissement of Paris.

French colonization of Indochina, (and more generally their efforts in south-east Asia: the problem is actually larger than your question, because if anything French is even less spoken in Thailand than in Vietnam right now), very much relied on help from “mercenaries” coming from various nationalities — apart from the obvious Portuguese, Italians, there were also Greeks, Japanese, and Chinese involved.

French involvment in south-East asia began during the late 17th century, notably with Catholic missions and diplomatic overtures to the Ayutthaya kingdom in the 1680s-1690s. Since this was a time of hot rivalry between the English and French navies, force projection over the Indian ocean was a bit hard for the French, who concentrated most of their force in the Mediterranean, so they relied on either cover as a church mission, or pure diplomatic privateering. This includes figures such as Konstantinos Phaulkon (born in Cephalonia, who came to basically rule the Ayutthya kingdo while being on the French payroll) or Alexandre of Rhodesia (who translated Vietnamese to Latin).

Later on, during the 19th century, the French did not suffer as much from communication problem as from a demography problem. Since the order of priority for settling French overseas territories was basically given by distance, with Algeria being first, and New Caledonia being last (hence them getting the least desirable emigrants, the convicts, but I digress). Indochina, being very far from France, was quite low ranked, and got settled by a hodge-podge of Italians, Bretons, Poles, Romanians, as well as some “undesirables” from other French colonies.

Meanwhile, the Third Republic enforced uniformity on its territories, and this included setting up schools to teach French everywhere, including in Indochina where very few (if any) actual French were present. The teachers were thus selected amongst the more linguistically competent of the foreign settlers, who happened to be mostly Spanish (hence “parler français comme une vache espagnole”) and Polish (in particular Polish Jews: “c'est de l'hébreu pour moi”), and the French taught in Indochinese schools started to slowly diverge from metropolitan French.

In particular, the Latin alphabet was introduced to Vietnam by Catholic missionaries who were not too used to French language, and got the various diacritics mixed up. This is the origin of the diacritics in modern Vietnamese: they were borrowed from French (acute, grave, circumflex), Romanian (breve), Polish (ogonek), or even Portuguese (tilda).

As this started to be quite a mess, in 1909 Governor-General Antony Wladislas Klobukowski decided to uniformize the alphabet; to keep everybody satisfied, in quite typical French colonial way, this was done by allowing all of the accents. Of course this did not improve much the mutual intelligibility between Vietnamese French and standard French (there is even some research on the fact that the Klobukowski decree might already have been misunderstood by the teachers).

By the start of Japanese occupation in 1940, Vietnamese French was already very divergent from French, as is shown by the Vietnamese orthography of words such as feu (Phở) or carotte (Cà rốt). As the Japanese tried to integrate Indochina (now the Empire of Vietnam) in the Coprosperity sphere, they finally instructed emperor Bao Dai to strike the “French” name from the Vietnamese French variant, henceforth known simply as “Vietnamese”.

In short, modern Vietnamese language is a very deformed descendant of early modern French language, but traces of French are still visible in its Latin alphabet, diacritics, and in some words.