Could you please provide a sample of the Vietnamese fonts you were trying at firstname.lastname@example.org? For me, writing ế, ệ, ằ works out of the box, even with Open Sans.
Which version are you using, web or desktop, and on which system? Could you please check if you have the latest version (3.2.6) installed? You can see the version from the welcome screen, when no design is open yet.