OmniWeb is best at viewing Vietnamese webpages; Safari, Chimera, Mozilla, and Opera work, too, while Internet Explorer and iCab don't.
Lucida Grande font is good for displaying Viet characters. Common Windows Unicode fonts, such as Times New Roman and Arial, work very well also. They can be downloaded from http://sf.net/projects/vietunicode and imported into OS X.
For more info about fonts in OS X, read http://www.alanwood.net/unicode/fonts_macosx.html .