I've used Pimsleur to learn Spanish, Thai, and Vietnamese, and Japanese (to varying levels). I will note that I have not used Pimsleur ESL for Vietnamese specifically (as I'm an english speaker), but instead would like to purchase it for a colleague of mine to use. Unfortunately the ESL lessons only go through 30, which generally is enough to have a minimal ability to navigate in that language (but not enough to hold simple conversations).
I have had experience with 4 languages taught by Pimsleur and have found it to be one of the best ways to learn to speak and understand a language quickly. The lessons are in half hour segments and you can repeat one as many times as is necessary until you can handle 80%+ of the lesson (after lesson 10 you'll be surely be repeating lessons a lot, they get hard, but fun).
The lessons move at full native language speed and take advantage of a lot of research into memory and natural language. The approach can be most simply summed up by saying that you learn in a way very similar to the way a child learns a new language. There isn't any dependency on rote memory techniques or traditional styles of learning lesson A, then B, then C. Here you learn by example, very carefully chosen examples which allow you to understand and learn the rules and logic of a language without expressly being told what those grammatical rules and structures are (nobody explained that stuff to you when you were a 1-year old, right? But you still got it).
The genius of these programs is truly in their simplicity.
