Well, I've met some unemployed locals who are desperate to get jobs.
One business owner (who happens not to pay the foreigners he hires) sells the dream of future earnings to make Vietnamese work for free. Other jobseekers paid millions to get some other job...
Some businesses love advance payments by foreigners. Then they have contract clauses which allow them to keep the unpaid salary in case "the director won't agree to the employee's notice to leave". This has worked out beautifully. Who would recommend some lengthy legal battle over a few weeks' salary?
Yesterday, my successor showed me the text she got. And it contained the same phrase the boss had used to me to justify non-payment for time worked. Rinse and repeat