If you would like to work as a translator or interpreter in Australia, you need to undertake a course that will give you accreditation with the industry body, the National Accreditation Authority for Translators and Interpreters (NAATI).
Interpreting courses are only offered at the higher levels, including Diploma, Advanced Diploma, and graduate or postgraduate degrees. At the level of Advanced Diploma or higher, you will be recommended for accreditation as a Professional Translator or Interpreter, while a Diploma course will enable you to work as a Paraprofessional Interpreter or Translator.
Royal Melbourne Institute of Technology offers Translating and Interpreting - Advanced Diploma (NAATI Professional) but you will need to contact RMIT directly by filling out the enquiry form to find out if Vietnamese is one of the languages offered on the course. In Perth, Central Institute of Technology offers an Advanced Diploma of Interpreting.
TAFE NSW – South Western Sydney Institute also offers the Diploma in Interpreting course, which would enable you to work at a paraprofessional level.
Hope this helps,