From Gero Herrmann:
The default Vietnamese keyboard layout of Mac OS X follows a National Standard of Vietnam, numbered TCVN 6064:1995. When typing with this keyboard, Vietnamese text is generated according to the Unicode encoding standard.
Vietnamese Keyboard Set provides additional keyboards with both alternative typing methods and alternative encodings for Vietnamese text to be generated. The alternative typing methods are Vietnamese-Telex, Vietnamese-VNI, and Vietnamese-AZERTY, a variant of the Vietnamese-Telex typing method based on the French AZERTY keyboard. The alternative encodings provided are ABC, ISC, VNI, and VPS. Among these, ISC is for use with ISC fonts such as Adobe Garamond Vie, while VNI can be used with VNI fonts such as VNI-Times, either in Macintosh or Windows format. The ABC encoding is suitable for typing with ABC fonts such as .VnTime in some Unicode-aware applications. The VPS encoding allows typing with VPS Macintosh fonts such as VPS Times.
In many Mac OS X applications, several text services for Vietnamese text are provided via the Services menu, which convert Vietnamese text among different encodings and provide lexical sorting according to the exact conventions used in Vietnamese dictionaries. For encoding conversion via the command line, an enhanced Macintosh port of UVConverter 1.1.3b, the universal command-line Vietnamese text encoding converter by Pham Kim Long, is included.
Vietnamese Keyboard Set further includes Nom-Telex, Nom-VNI, and Nom-VIQR input method plug-ins, which enable typing of the traditional Vietnamese Nom characters on Mac OS X, e.g. in combination with the freely available NomNaTongLight font.