Well, apart from it being named "Dong" it is also the currency with the highest numbers on its bills. Worldwide.
There is a 500,000 Dong bill and it is not an exotic one like the 500 € bill, that you hardly ever came across in everyday life. Half a million Dong will only get you the equivalent of (roughly) 20 € or $. So usually, after coming back from the ATM, you will be a multi millionaire in Vietnam.
That seems cool first, but is really confusing in reality, because locals hardly ever say the complete number when talking prices. "Two hundred thousand" just becomes "two hundred" and might make you search for a bill so small, it does not even exist (you might not see anything smaller than 1,000).