Several years ago, I paid $$$ for a full set (like 21 cd's) from Pimsleur for Vietnamese ahead of a 3-week vacation to that wonderful country.

Almost useless as a screen door on a submarine. The CD's are NORTH Vietnamese - which shares suprisingly little with SOUTH Vietnam (which is where I went).

Vietnamese as a language was once symbol-based like Chinese, Japanese or Korean. In the 17th century, written Vietnamese was converted to the current Romanized version in use today. The problem with the conversion is that it didn't follow traditional Roman/latin pronounciation rules. The discs even go so far as to warn you not to attempt to read Vietnamese while learning Vietnamese. For example, D's are pronounced like Z's (sometimes) TR is pronounced CH (Nha Trang, a beautiful seaside city is pronounced Nya Chang). The lanuage uses every diacritcal mark known to mankind to "aid" you in pronounciation.

I did find on the internet once-upon-a-time a dated (from the 60's / 70's) a state department document that taught conversational Vietnamese for the south. Learned more from that document than I did from 21 cd's.

I think for latin-based languages it would be no problem. You can bolster your language skills by reading the printed word once you get a good basis in the language from the audio.

Not so with Vietnamese.