'bo*' is pretty common for cows in european languages - they are the genus 'bos' after all (see: english 'bovine').

As for the Vietnamese roots, I thought it might have been a French loanword but it looks like it came from a proto-vietnamese language: https://en.wiktionary.org/wiki/b%C3%B2#Etymology_1

The Google etymology viewer is pretty good for this: https://www.google.com.au/search?q=etymology%20cow

I have just made the most shallow research, but in Swedish (and other Scanian languages) cattle are called "boskap". The etymology is shared with "bo", meaning dwelling (animal or human house). The original meaning of "boskap" is that which belongs to the house.

I'm wonder if it might be shared with the other languages, that cows and sheep share the name of the dwelling because they are part of it?

Looking further at the etymology of (Swedish) "bo", it seems it comes from the Proto-Indo European "bhuh", to become, to grow into being, sharing the same etymology as English' "be". In Swedish, "bo" is also a verb meaning "to live (some place)", e.g. "I live here": "jag bor här".

Edit: Actually, "be" has a mixed history it seems. Indeed it is mixed with "to reside". See here for more etymology: https://en.wiktionary.org/wiki/be#Etymology

Edit II: After a bit more of Wiktionary, "bo" has the exact same meaning as in Swedish in Cuiba, a language with less than 3000 speakers in Colombia and Venezuela. Wow!

When it comes to the cows though, my original research might have been misguided. Looking at the etymology for Latin "bos", it comes from a different Indo European root: https://en.wiktionary.org/wiki/bos#Latin

French had a big impact on Viet culture and I believe were in some part related to the romanization of the language. To me Viet seems like Cantonese written in latin script, with borrowed French words. I don't know it very well obviously.