I recently started the new Vietnamese program on Duolingo. Because the program is still in beta, it doesn’t have all the functions that most other programs do. For example, you can’t scroll over to pronounce one word at a time, only the full sentence. Also, when writing a response in Vietnamese, the site does not yet include buttons for accented characters like it does for other languages (ç, ß, ñ…). While the typical keyboard shortcuts cover a few Vietnamese vowels (ô, è, á…), many others are just not possible with the English keyboard (đ, ự, ấ, ẽ, ễ, ắ…). For now it is necessary to turn on the Vietnamese keyboard on your mac. This can be done by clicking the American flag in the top right corner of the screen, going down to Open Keyboard Preferences, hitting the + button at the bottom left, and selecting Vietnamese from the list. Then, click the flag again and select Vietnamese. Click the (now Vietnamese) flag and hit Show Keyboard Viewer to open a small window which both acts as a keyboard itself, and shows how different letters correspond to your keyboard keys now.
In order to type a vowel with multiple accent marks you will have to first select a base, after which the keyboard will change to show multiple options for how to add on to it. Good luck!!