[wittekm]

'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

[nl]

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