I've had good luck configuring the correct font sets for CJK languages by modifying the tfontsub file, ie., Serif/jpn -> DFSMGothic-Lt.
But now I need to modify the font set for Vietnamese. Adding something like this doesn't seem to work: Serif/vnm -> Arial. Arbortext is rolling to my standard serif font first and then plugging in the default font. Can I add other languages to the tfontsub file?