It wasn’t such a bad fit for Vietnamese, whose syllabic structure is basically the same as Chinese languages. Actually Han and Chu Nom were used for a way longer time than Quoc Ngu is. And the change to a latin based alphabet is more due to colonization than anything. For Korean you right though.
The absurd number of diacritics one must append to Latin letters in order to use that alphabet for Vietnamese is the best evidence that Vietnam should have had its own rationally constructed system like Korea.
