http://vietunicode.sourceforge.net/charset/quytacABC_en.html is documentation from the Vietnam Lexicography Centre about Vietnamese sorting rules for dictionary entries. Thanks to Pavel Striz for supplying that information at 2014-03-28 in the xindy mailing list and also sending a patch that fixes the former incorrect sorting. Comment from Joachim Schrod at 2014-04-19: I have not checked if rule 4 and Note 1 from that documentation are really realized by xindy. If somebody could check and tell me, I would be grateful.