Hello and welcome to the wonderful world of debian-l10n/i18n! The basic components of the translation work to be done are the po files of programs and po-debconf files. The regular po files contain the strings that you see all over the Debian system, such as the login prompt etc. These are used by the programs.
Po-debconf, however, contains strings that are used by debconf while configuring the packages, e.g. while installing the package or whenever you run dpkg-reconfigure. For a beginning translator I would recommend starting with po-debconf, because they generally contain less and easier to translate strings (not all of them though...) in contrast to the po strings which are generally more specialized... You should probably meet up with Clytie Siddal, who for as far as I know currently is the only one active for the Vietnamese l10n project. Her address should be somewhere in the list archives over here... The packages that are listed under "already i18n-ed" have been prepared for internationalization, but haven't been translated to you language yet. You should create new po files from scratch, based on the PO Templates - POT files. You can find a link to these at the top of the translation status pages, both for regular po and po-debconf. I hope I have helped you into the right direction and good luck with translating! Regards, Eric Spreen PS Maybe this would be a good motivation to reconsider opening a debian-l10n-vietnamese mailing list?
