Purchase a CD that suits your abilities. If you are just starting out learning Vietnamese, then it's a good idea to invest in a CD that contains just basic, introductory phrases; if you are an advanced student, then more complex dialogue will be needed if you are to improve your skills in your sleep.
Set your CD player's volume low enough for you to be able to fall asleep yet high enough for you to hear through the night.
Set your CD player to "repeat" mode. In order to get the most benefit out of subliminal learning materials, the content must be reinforced. So if your Vietnamese language CD is an hour long, you'll have heard the same words and phrases eight times by the time you get up (provided you enjoy the recommended eight hours of sleep).
Repeat the process for several days or even weeks. Learning a new language takes time and repeating what you learn plays a key role in picking up new phrases and expanding on your vocabulary. So don't be disappointed if you don't see any great difference in your Vietnamese skills overnight, but instead stay focused on your longer-term goals.
Test your memory retention once you wake up. Most learning aid CDs will come with transcripts or even little quizzes, and these can be a good way of measuring your progress and deciding whether you are ready to move onto the next disc or if you need a few more nights with your current one.
Complement the subliminal learning with daytime activities. While being able to remember a list of new words is helpful, it's also important to practice them out loud, so try working with a study buddy or even just speak to yourself during the daytime to reinforce your nocturnal education.