$USD ?

I think it's the AU$ that is fluctuating.  It depends which direction you are looking from.  I have read that the AU$ is very dependent on commodity prices, particularly coal and iron ore.  I don't know about iron ore but coal is definitely down worldwide.

Maybe all those $100 bills in the hands of the Vietnamese gold shops serve as a stabilizer.   

I have read that the Vietnamese government sets the exchange rate against the various currencies,  which is unlike many countries that utilize a free market for currency valuation

yes I suspect that,

Vietnamese central bank sets daily exchange rate
English.news.cn   2016-01-04 19:29:41       More 
HANOI, Jan. 4 (Xinhua) -- Vietnamese central bank on Monday started listing inter--bank average exchange rate between Vietnamese dong (VND) and U.S. dollar (VND) on a daily basis, instead of fixing a rate for a long period of time as before.

Accordingly, the inter--bank average exchange rate on Monday was set at 21,896 VND for one USD, higher than the previous rate of 21,890 VND/USD fixed from Aug. 19, 2015.

The daily--adjusted rate will be "the basis for credit organizations and branches of foreign banks, which are allowed to provide foreign currency services, to decide buying and selling rate between VND and USD", the State Bank of Vietnam (SBV) said on its website.

The new rate calculation will be based on the exchange rate changes in the inter--bank foreign exchange market, monetary developments in international market of foreign currencies which are involved in trading, investment and financing with Vietnam.

The new policy will enable the exchange rate to be flexibly adjusted following domestic demand on foreign currency and global monetary fluctuation, while still maintaining the managing role of the central bank, said the SBV