As seen on that page, they use too much of invented or re-assigned characters that maps to native Vietnamese vocabulary. Sentence structures also differs too mu

As seen on that page, they use too much of invented or re-assigned characters that maps to native Vietnamese vocabulary. Sentence structures also differs too much. As implied in a sibling comment by karmasimida, we CJK speakers have a bit of mutual text-only intelligibility across CJK - speeches are all steelpans unless you have few years of training + auditory cortex power cycling, just writing - and that completely breaks apart in Vietnamese, in the way very reminiscent of looking at Cyrillic script.