First, make sure that you have Mac OS X 10.3 (Panther) and the latest version (1.3) of the Vietnamese keyboard layouts.
To type with the VNI keyboard layout after installation, you have to activate it in System Preferences->International->Input Menu. Check the box next to "Vietnamese-VNI". I also recommend to check "Keyboard Viewer" because this will help you to learn to type with different keyboard layouts. Whenever you wish to type, select "Vietnamese-VNI" in the input menu. To see a graphical representation of the layout, select "Keyboard Viewer" from the input menu as well.
MS Word 2004 (not the older MS Word X) is compatible with Vietnamese Unicode and the Vietnamese-VNI keyboard layout. About InDesign I do not know. Photoshop 7.0 is not compatible.
You still may type Vietnamese in non-Unicode applications such as Photoshop 7.0 or MS Word X if you use VNI or ISC fonts as opposed to Unicode fonts. Version 1.3 of the Vietnamese keyboard layouts package includes two layouts, "Vietnamese-Telex (ISC)" and "Vietnamese-Telex (VNI)" for this purpose. As the name says, both are Telex-style layouts, not VNI-style. If you are interested in a VNI-style layout for either ISC or VNI fonts, please let me know, and also which of the two encodings you would like to use.