The default is currently only female, but quite good. With voice input, you can plug in headphones to get better navigation when moving. This version is still not high quality (Hi-fi), hopefully in the future, Nokia will soon update it to make you hear better.
To download Vietnamese voice, go to Settings -> Voice navigation -> Manage voices -> drag down with Vietnamese section -> click on it to download. In case you haven't downloaded Here Maps for Android, download it here: http://here.com/download