Despite government attempts to outlaw the practice, the USD still acts as an alternative currency which is almost completely interchangeable with the dong. Many prices, especially for hotels, tours and expensive restaurants, are still quoted in US$, though you can pay in dong if you’d rather – just check what exchange rate they’re using.
For everyday expenses, I recommend carrying a mix of US$ cash and dong. For larger items (hotel bills, train tickets, etc.) or when the exchange rate works in your favor, use dollars. For cyclos, local food stalls and small purchases, it’s best to use dong. In either case, make sure you always have a stock of small notes so that you don’t have to worry about change.