You need to have Vietnamese fonts installed in your systems in order to be able to see Vietnamese characters. Fonts are designed specifically for each character set and are generally not interchangeable. For instance, if you use VISCII charset, you need VISCII fonts. If you use VPS charset, you need VPS fonts. If you use ABC character set, you need ABC fonts. Unicode supports characters from many languages, including Vietnamese. Thus if you use Vietnamese in Unicode charset, you must use Unicode fonts.
Following are a few web sites that offer free font download:
VISCII fonts:
http://winvnkey.sourceforge.net or more specifically here
http://www.vn.net/download.php
http://unicode.sourceforge.net or more specifically here
http://vietunicode.sourceforge.net/ or more specifically here
VPS fonts:
ABC fonts:
Le Hoan Website (VISCII, VPS, VNI, ABC, DHBK 1/2, VietWare, Viet Unicode )
VNI fonts:
Unicode fonts:
http://www.microsoft.com/vietnam or more specifically here
http://vietunicode.sourceforge.net or more specifically here
http://www.vn.net/download.php
Phat Hoc Dao Uyen or more specifically here
Unicode Fonts for Romanized Viet-Pali-Sanskrit (chữ Phạn)
http://www.babelstone.co.uk/Fonts/Fonts.htm
Unicode fonts supporting surrogates:
http://www.babelstone.co.uk/Fonts/Fonts.html#Code2001, or more specifically here