Hi Sarah,
I did this trip in November and as Vickie says then in Cambodia (& Laos) the US$ rules.
I took cash, as I think did most on the trip, so did not use ATM's.
If you are paying for something you frequently get local currency in your change but don't worry as on your next transaction the seller etc will usually take your local money and then make up any difference in US$.
It is a great trip.
However, if you are doing the Hanoi add on then the dong rules but you could change money in the hotel & there were plenty of money changing places in Hanoi itself.
Ian