$AUD = VND21,700
Rough rule of thumb if you are short of cash is to use an exchange rate of VND21,000 / 1AUD
Cab drivers would happily accept Aussie cash if you do not want to use the exchange desk at the airport.
I personally use the various No-Commission exchange desks dotted all over the city. So many of them. At every corner.
Better rate, no commission and no paperwork - you don't even have to show any ID (unlike the ANZ branch I walked into, only to walk out soon after).
The number of merchants accepting MasterCard / Visa cards is a real surprise to me - very, very widespread.
Posting from the Caravelle Hotel sipping Bailey infused ice creams.