In order to type Vietnamese characters on your Mac you will first need to set Vietnamese as one of your preferred languages. To do this go into your System Preferences, select the Language & Region section and hit the plus symbol to add Vietnamese as a preferred language (as per the screenshot below).
Next you will have to select the language in the menu bar so that you can type the Vietnamese special characters.
Now, with the language selected you will be able to type the regular roman characters as well as some Vietnamese ones. To type the special characters use the number keys on your Mac’s keyboard — so if you press 1 you’ll get ă, if you press 2 you’ll get â etc. The graphic below will show you which numbers produce which characters.
If you want to type numbers while your Mac’s keyboard is set to Vietnamese you can do so by holding down the alt/option key while pressing the number.