I am an Australian living in USA. I am also a green card holder. I want to travel to Vietnam by the end of this year. Should I use my Australian Passport or I have to use my green card with re-entry permit ? Thank you for your answer.

Hi Duy,

You will need a tourist visa on your Australian Passport to go to Vietnam (source). Since you are on Green card and do not hold an American passport yet, the only thing you can use is your Australian passport. But carry your Green Card along with you as that will be needed to re-enter USA on the way back.

Hope it helps
Have a safe trip!