AHC: Vietnam expanded west (to the east of Mekong River)

The expansion of Vietnam throughout most of its history could be described as "Champa-screw" (with a little bit of Cambodia-screw for the Mekong Delta) as the former expanded its territory southward through conflicts and/or treaties with its (former) Indianized Austronesian neighbor.

I was really thinking recently of an alternate Vietnam that expanded westward to the east bank of the Mekong River instead of expanding south; what POD should be needed to realized it?

What would be the possible impact on the following:
1. Tai migration to present-day Thai territory, especially in the Isan region.
2. The settlement of the northern tip of Sumatra.
3. The effects of a surviving Champa on the history of (continental) Southeast Asia.
4. Impact on the other Austroasiatic groups living in OTL Laos.

Share your thoughts, guys.
 
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Geographical or ethnocultural? Because somehow Kinh is poor at spreading itself into mountainous regions, only occupying plains and coastal regions.
 
I'm thinking of a geographical expansion, but ethno-cultural can also be (possibly) viable, I think

Kinh, IOTL, expanded in correspondence with plains, unlike the also agricultural, city-dwelling Han who fully colonized Southeastern China. I think there's some less-than-obvious cultural differences between the two that caused such difference, or just that Indochinese natives are more fiercely independent-minded than those of Southeastern China?
 

Sorry for the late response, but I share the concern with kasumigenx: the Qin/Han era use of the word "Yue" was very broad, and consists of modern Yue(Yuht, Sino-Tibetan), Min(Man, Ming, Ban, and others I don't know how they read "Min", Sino-Tibetan), Hlai(Tai-Kadai), Zhuang-Tai(Tai-Kadai), and, finally, Kinh/Viet(Austroasiatic). In that scenario, since the polity originate from the natives of Guangxi, it would be a Tai-Kadai state rather than "Vietnamese", though it might be labeled as such by the ATL Han and became gradually accepted by their people.

And, honestly, I don't think anyone is certain about who used to live in the modern Min and Yue-speaking regions before Han arrival. Were they Tai-Kadai or Austroasian?(neighboring cultures) Other Sino-Tibetan cultures?(explains the absurd lingual diversity from, supposingly, a single source 2,000 years ago, all while political united, which is almost ASB) Or perhaps Austronesian?(Dabenkeng culture = proto-Austronesian hypothesis)
 
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