To give insight into the last comment there:
Although Unicode does provide precomposed characters for most of the letters used in Vietnamese, the default keyboard in Windows DOES NOT generate precomposed characters.
For example, while in French, one uses dead-keys, type in ' then e and it generates the pre-composed character E WITH ACUTE ACCENT é
Vietnamese keyboards by default just output the combining mark in isolation.
So the keyboard 5, 6, 7, 8 keys generate COMBINING DIACRITIC MARKs which can be entered in isolation, in addition to repeatedly, or after consonants or symbols.
Thus, to properly support Vietnamese input, fonts are expected to support ligatures transforms that map the valid sequences to an optimised precomposed glyph; as well as providing anchors for combining diacritics for applications that cannot handle ligature rendering support.