Chrome Version: 71.0.3556.0
OS: 11042.0.0

In strict mode, all Vietnamese IMEs require diacritic triggers to immediately follow the letters that need diacritics. 

There's a popular flexible diacritic trigger mode where diacritic triggers can be typed "later" (towards the end) or "earlier" (towards the front). The IME is supposed to place the diacritic on the most linguistically correct letter as per the orthography at every step. "Vietnamese keyboard (VNI)" on CrOS does implement the flexible mode (albeit incomplete, see crbug.com/890218), but not behind any settings flag.

As the flexible mode relies on Vietnamese orthography, there's preferably a user settings flag to guard it, so that user can force strict mode and use the IME purely as a way to expand their character repertoire with those present in Vietnamese, but not necessarily to type orthographically correct Vietnamese tokens. E.g. proper names (foreign, or minority-language) like those of places, people, brands, etc., and loanwords, or simply neologisms. 

VNI rules have facilities to cancel diacritics, but if mixing non-Vietnamese tokens is a common use case, then the user may find strict mode a better choice.