From Just Solve the File Format Problem
Windows 1258 (CP1258, Windows-1258, Windows CP1258, Windows Vietnamese) is a character encoding used in Microsoft Windows systems, aimed at Vietnamese. It is one of the Windows encodings. It is similar to Windows 1252, but differs in a few points. It has some combining diacritics intended to be used along with other characters to get the wide range of letters with diacritics used in Vietnamese.