Perhaps we can go back in history a little bit further.
The name Vietnam most probably based on Nanyue or Nam Viet a small little known kingdom created by Qin Dynasty deputy General, name Zhao Tuo or Tro Duo, a Han Chinese [1]. The indigenous or the native people of central and south Vietnam are not from China, they are mainly from Austronesian origin probably Taiwan so called Chams or Cham people [2]. They were the main inhabitants of the land for more than a thousand years (around 1500 years) and had a flourishing Champa kingdoms from 2nd to 16th century AD. The Chams mainly Muslim at the time, had been evicted just like the Moro in the Spain during the Spanish Inquisition or forced to convert. The eviction is completed after the Kinh people from the north displaced them during the Nam tien or "March to the South" and by doing that essentially tripled the original northern kingdom area size [3]. It's a kind of ethnic cleansing by today's standard.
The Kinh is originally ethnic Chinese intermarriage with local Tai-Kadai of the northern Vietnam. For about a thousand years they were essentially conquered and ruled by imperial China. As an analogy the Chams is like the Maya people of Vietnam, until they were eventually evicted rather than assimilated in around 16th AD by the Nguyen lords from the north to the other surrounding countries/areas towards the end of Nam tien movements.
The Chams has a proper civilization called Champa just like the Mayan civilization. The Chams' language is the pre-cursor to the modern Malay and Indonesian language. Their oldest writing on a stone tablet namely Dong Yeng Chau contains an old classical writing of Chams languages in the 4th century AD and was discovered recently in modern Quang Nam province, central Vietnam. It is amazingly still comprehensible if you know the modern language [4]. Interestingly the old name for Quang Nam is Simhapura or Lion City, and it is highly probable than the Cham people conquered Singapore (Singapura) and renamed it similar to their capital's name just like York in the UK becomes New York in the US.
[1]http://eastasiaorigin.blogspot.com/2018/05/ethnic-origin-of-...
[2]https://en.m.wikipedia.org/wiki/Chams
[3]https://en.m.wikipedia.org/wiki/Nam_ti%E1%BA%BFn
[4]https://en.m.wikipedia.org/wiki/%C4%90%C3%B4ng_Y%C3%AAn_Ch%C...