You need to have fonts with Vietnamise characters. For example Minion Pro. That is all.
You can convert this text in any good text editor, which has the option "Reopen using encoding", For example Text Wrangler.
Yu can convert every Vietnamise glyphs using grep in InDesign too. But it is very time consuming job.
Thanks Olfar for this - Ive looked at Minion pro and its a serif font. Do you know a font that is sans serif t. Once agin thanks for helping mehat I can use. Ideally I would like to use helvetica light, but I'm unsure if this has all the characters in. Once again many thanks. Lisa