Is there a theory challenging the "strict" distinction between Thai and Vietnamese?

Question

I understand Thai and Lao and all their dialects, and Vietnamese and all its dialects to be of totally different language evolutionary families (Tai Kra-Dai and Astroasiatic).

I can speak and read some basic Thai and have visited Vietnam and my opinion is that both languages are very different;
When I just started to learn about them they sounded to me extremely similar but with time extremely different.

But I am not sure it is correct to say that there aren't any significant commonalities at all:

• Both are significantly tonal
• I am impressed that both have lots of B, T and L.
• I think that both languages have large usage of consonant-vowel combinations
• I am impressed that both share, humbly, to my ear, very similar yet not identical quite unique phonemes, such as:

Sound comparison 1: Both share the rare ng consonant (ง in Thai) | available in Nguyễn in Vietnamese
Sound comparison 2: ◌ู in Thai for uu sound | m(ườ)i for a similar uu sound in Vietnamese
Sound comparison 3: เออ in Thai for an impolite agreement between close friends only | A similar way to agree impolitely(?) in Vietnamese (as far as I have recognized)
Sound comparison 4: มื in Thai for m(ue') sound | Shouldn't it be similar to, for example, the ue' in Tú Xương?
Sound comparison 5: The Issan (Thai-Lao) way to pronounce the word Eighty | A similar way in Vietnamese (as far as I have recognized)

When I learn of Issan (the geographically closer part of Thailand to Vietnam) architecture, furniture, musical instruments, traditional food and clothing I do start to think that common effects might were more common than it may seem. Issan Language itself might be closer.

Using a metaphor I would say that these are two different buildings from the same materials (same proto language perhaps? "proto south east asian" perhaps.

Is there a theory aimed to doubt the common strict distinction between Thai and Vietnamese showing that Proto Tai Kra Dai might have been effected from Vietnamese or vice versa, more than commonly thought?

Answer 1

There is a theory, applicable to all human languages, that is even encoded in what certain words mean in linguistics. Namely, "related" is taken to be a claim about genetic (historical) relations between languages. When we say that English and German are related, we mean that they historically derive from a single language. When we say that English and Finnish are not related, we mean that there is no (identifiable or reconstructable) language that is the common source of English and Finnish. That does not preclude observational similarities (English and Finnish are structurally more similar that English and Chinese or English and Tubatulabal).

Two languages which are genetically unrelated (or only very distantly related) may nevertheless end up looking like each other to some extent, especially in terms of superficial sound similarities. This is the domain of areal linguistics / contact linguistics, and there is a word for this, "Sprachbund". For example, Modern Armenian morphology looks a bit more like Turkish, for reasons of significant language contact. I won't review your specific proposal regarding Thai and Vietnamese (being tonal is normal, albeit not in the most widely-spoken languages – probably a majority of human languages are tonal). I agree that there is something about Thai and Vietnamese where they sound similar.

The explanation, IMO, is that people are able to pick up in broad features of "how a language sounds", and emulate their neighbors, if they want.