If you're calling from Vietnam then you need the international dialling code 00 then it's 61 for australia then you drop the first 0 of the mobile number. I assume you might have to do the same if calling from a vietnamese mobile within Australia. It would work anyway. about 10 years ago
http://www.countrycodes.com/ guides you through calling internationally from any country.
+6104 09 000 000
would be 00614 09 000 000
Every country has a different international prefix to be able to ring OUT of the country. Vietnam uses "00".
Most also allow the use of a "+" instead of their prefix but some might not. about 10 years ago