How much does it cost in that location now compared to what it's going to cost now to make it in Vietnam?
How many Vietnam workers is this going to employ, and are they going to be working for like... $3/day?
So... a lot of people.
'Money is the first thing. Morals follow on.' (based on B. Brecht)
From Wikipedia:
Democracy Index 2021: Vietnam 131 / China 148 (Norway 1 - best / Afghanistan 167 worst)
Freedom In The World 2021: Vietnam 167 / China 184 (Finland 1 - best / Syria 194 - worst)
The actual thing is not really poor population, but how easy is to build new factory there.
Vietnam has the best thing for that - it is easy to build new stuff there, as the infrastructure exists; however, they absolutely ignore any environmental laws and just spew all waste into rivers and air.
Look up Formosa Steel incident - https://en.wikipedia.org/wiki/2016_Vietnam_marine_life_disas... - huge disaster of a Taiwanese company, after which there were big protests. Communist government then arrested all protesters and put them to 20 years in prisons (for protesting against a foreign company!). Nobody protests about environmental waste now.