ATo add the ability to display most foreign languages on your computer you need to install a font for that language. This is true for Windows 3.1, Windows 95, Windows NT, OS/2, Mac OS and Unix. Some languages require the installation of special software. These include Chinese -- which uses a separate character for each word rather than an alphabet -- and Russian, which encodes its alphabet using Soviet era encoding schemes that are incompatible with most standard versions of operating systems.
In the past, Vietnamese was written using Chinese-style characters. When the French were the colonial power in Vietnam they created an alphabet for the Vietnamese language which is based on the Latin alphabet -- the alphabet used by languages such as English, French and Spanish.
For computers using the English language we mostly use the ASCII encoding scheme, other schemes having long since disappeared or confined to obscurity. With Vietnamese there are two main encoding schemes, and therefore two alternative font sets that can be loaded on a computer to allow Vietnamese to be read. VPS fonts are probably the most widely used, but VISCII fonts are also fairly common. There is a third encoding scheme called VNI, but is not often used.
To allow others to read your Vietnamese documents, include links to the appropriate font packages for their computer. VPS and VISCII fonts for a variety of platforms can be found at ftp://ftp.media.mit.edu/pub/Vietnet/. People who can read Vietnamese will likely already have fonts for VPS and VISCII on their computer. Others will need to download and install the necessary fonts for their system.
The HTML tag to specify a font for, say, VPS Times is <FONT FACE="VPS Times">. You can never be sure of the exact fonts the user has on their computer, though people using VPS fonts will probably have the standard VPS font set. Using the FONT FACE tag will make it easier for users. Leave instructions to switch to Vietnamese fonts, as it is likely that this automatic switching will not work for all users.
Creating Vietnamese Web pages is fairly straightforward. Given the number of characters in the Vietnamese alphabet (or more accurately, the number of stresses that can be applied to the letters) you need software that allows you to type in Vietnamese on a standard keyboard. There are 2 options. If you use VPS fonts on Windows they are generally distributed in a bundle called VPSWin, which includes a program called VPSKeys. This allows you to type in Vietnamese in any Windows program. If you wish to use VISCII, you have to type the Vietnamese text in a plain text (ASCII) version of Vietnamese. Then you run a program called VIET78X (available from ftp://ftp.media.mit.edu/pub/Vietnet/Viscii/Dos/) that will convert the file to VISCII.

VpsKeys lets you type in Vietnamese in any Windows program