Mac users running OS X Leopard (version 10.5) or a later version should have built-in support for Vietnamese. Just follow these steps (but keep in mind that the naming of various user interface elements may vary from one Mac OS to another):
1. From the Apple menu, select System Preferences.
2. Click on International.
3. In the International window, click on the Input Menu tab.
4. In the Input Menu tab, select Vietnamese UniKey.
(By the way, your current keyboard should be kept selected.
Vietnamese UniKey, select the input method you want to use: Simple Telex, Telex, VNI, or VIQR.
6. Depending on the Mac OS X version you are running, you may or may not see the option allow a different input source for each document. An input source is basically a language and its corresponding input
method. If you want to be able to have different input sources active in
different applications, select this useful option. That way, you won’t
need to keep switching input sources every time you switch between
7. Select Show input menu in input bar to be able to easily switch from one input source to another within the
same application. The input menu will appear on the right side of the
menu bar and display all the enabled language input sources you
Tip: Using keyboard shortcuts allows you to switch between input sources faster
than by selecting the desired input source from the input menu. The
input menu should list what the default keyboard shortcuts are. You can
assign different keyboard shortcuts by clicking on the keyboard
IMPORTANT NOTE: If you've recently enabled Vietnamese typing and find that your usernames and/or passwords are no longer working, it may be because you are typing in Vietnamese. Read steps 5 and 6 above to learn how to change input sources. Alternatively, deselect Vietnamese UniKey in your System Preferences if you no longer need it.