The Vietnamese have traded their Nom script that was based on the Chinese Han ideographs for a Latin alphabet long ago.
The new Vietnamese alphabet of Latin letters with up to two accents is too large to fit into ISO 8859. The best encoding for Vietnamese is therefore Unicode (ISO 10646). If you cannot use Unicode yet and need to use an Vietnamese 8bit charset, try VISCII:
charset=VISCII [TXT] [BDF]
VISCII was developed in 1993 by the Vietnamese Standardization Working Group alias Viet-Std@Haydn.Stanford.EDU. Their FTP site and RFC 1456 tell you a lot more about VISCII and the Vietnamese Quoted ASCII Representation VIQR.
There are other Vietnamese encoding standards that require your computer to know how to combine accents onto variable glyphs to produce the less frequent accentuated capital letters: VN5712-1, VN5712-2 (VSCII, ISO-IR-180) and CP1258.
I have added a Vietnamese.kmap to the Unicode text editor Yudit for the X Window System.