It's not a transliteration. Vietnamese is written using Latin script. Because the modern script is still relatively new, there is a strong correspondence between the written form and the standard pronunciation. And because Vietnamese is not Latin, many odd-looking compromises had to be made to represent all the sounds using something resembling the Latin alphabet.
The situation with Chinese is similar. Pinyin is not a transliteration standard but a standard way of writing the Beijing dialect phonetically using Latin script.