This is the process I follow to replace the Avenir and Georgia system fonts with unicode versions that support Vietnamese
1. Download the latest firmware from
https://pgaskin.net/KoboStuff/kobofirmware.html
The firmware will look like "kobo-update-4.33.19759.zip"
2. Unzip the firmware to the folder "kobo-update-4.33.19759"
The folder will contain an "upgrade" folder, a KoboRoot.tgz and a md5sum file
3. Right click KoboRoot.tgz, Open with 7zip (or other unzip program)
4. IN the zip window, double click on KoboRoot.tar
5. Navigate to .\usr\local\Trolltech\QtEmbedded-4.6.2-arm\lib\fonts\
6. Copy your prepared Unicode Avenir and Georgia font files to replace the stock fonts Avenir and Georgia in .\usr\local\Trolltech\QtEmbedded-4.6.2-arm\lib\fonts\. You have to search on "How to prepare the unicode font files"
7. Save and close the tgz file
8. Select the 3 items (included the updated tgz) in kobo-update-4.33.19759 and zip them into kobo-update-4.33.19759.zip
9. Connect the kobo libra to the PC and copy the updated kobo-update-4.33.19759.zip to ".kobo" folder in the Kobo ereader
10. unzip the kobo-update-4.33.19759.zip in the .kobo (these 3 items have to appear in .kobo, not in a subfolder)
Note: somehow copy the 3 items directly from the PC to the Ereader .kobo won't work. I have to zip them first, then copy the zip file across and unzip the zip file, then it works.
11. Eject the Ereader and restart the Ereader, it should go through the update firmware cycle.
12. Done.