The difference between them is the tone, man. Different Vietnamese accents have different tones. It's quite hard to explain by just writing, you'd better learn it through listening (it's even better if you have someone who is Vietnamese practice with you). Anyway, you have no choice but try to distinguish them (by watching the video mentioned above). I know it takes time, but just be patient. Sooner or later you will make it!
Feel free to ask if you have any further question or even want me to help you with that problem. Good luck!