Thanks for your guide @astroner.
However, I got away with something simpler:
* I didn't need to use KoboRoot.tgz or root my device whatsoever.
* I didn't use your attachment tiengviet.rar.
* No terminal usage, I just used Finder on my mac.
But your advice to use Noto Serif was spot on and probably saved me lots of time! Again thanks!
If anyone finds it useful, the instructions in this thread are similar to the ones in this video:
https://www.youtube.com/watch?v=MahsySF4Hk4