@ Cornelio,
just try these agencies like Parc Aviation or Rishworth, they hire for Vietnam Airlines - at least pilots' jobs. They have a site for maintenance crew as well, maybe they have any job for you.
But you can also try any other of those agencies, I guess all or at least most of them support maintenance crews as well. You will have to register there, but don't hesitate to get in contact with them via E-Mail or phone and ask what you want to know.
Good luck!