Any other ideas?
A quick search shows that vietnamese words are sorted by letters first, then tone-marks. Sometimes tone-marks are even ignored. So that's probably correct: 
ỳ :
ỷ :
ỳ ạch :
http://vietunicode.sourceforge.net/charset/quytacABC_en.html ...looks like Unicode::Collate does it right, but additional first-character ordering is required to get a dictionary order (happens to be different from a simple sorted order in vietnamese).

In reply to Re^7: Sorting Vietnamese text by Atacama
in thread Sorting Vietnamese text by pdenisowski

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