two keyboards for typing Vietnamese
These two have all the special accents, which Google Input Tools doesn't have on its Vietnamese keyboards.
To use them, you have to type using your English keyboard and then choose the special Vietnamese keys, then copy and paste it into the box on Duolingo.
I think the usefulwebtool.com keyboard is better.
Take your choice.
Meanwhile, Windows 10 includes a Vietnamese keyboard. Thanks goodness, it is qwerty-based ̣(I expected a French azerty layout). One must note that the tones works like dead-keys, but should be pressed after the vowel ̣(different from dead-keys in European keyboard layouts).