Frommer's lists exact prices in the local currency wherever possible, though the U.S. dollar is used widely in both Vietnam and Cambodia: In fact, the dollar is the de facto currency in Cambodia, and packing some U.S. greenbacks will come in very handy. Rates fluctuate, so before departing, consult a currency exchange website such as www.oanda.com/convert/classic to check up-to-the-minute rates.
During your trip, the most useful Vietnam Dong bills will be upwards of 10,000 VND. There are smaller bills (which are also physically smaller than the more frequently used bills of 10,000 VND and up) of 1,000 VND, 2,000 VND, and 5,000 VND, which are handy when buying snacks from street vendors or if you want to give exact change to cabdrivers. Every now and again, a bronze 5,000 VND coin will land your way. For the most part, bills are distinguishable by color: The 500,000 VND is light blue, 100,000 VND is green, and 20,000 VND is dark blue. Be mindful of the 10,000 VND, 50,000 VND, and 200,000 VND notes -- all are done in pinkish-red hues that are quite similar to each other.
ATM service is good in most cities and the machines accept four-digit PINs. If heading off into the countryside, bring cash. Credit cards are also widely accepted, though many smaller companies, such as tour agencies or boutique hotels, will charge a 2% or 3% commission. For now, the traditional swipe credit cards are still widely accepted. All hotels can do business in U.S. dollars. In some parts, everybody down to the smallest shop vendor quotes prices in U.S. dollars, and particularly the big-ticket items are best handled with greenbacks instead of large stacks of local currency.
While dealing in U.S. dollars can make things less complicated, always keep in mind local currency values so that you know if you're being charged the correct amount or are given the correct change (usually in Vietnamese currency). In this guide, I've listed hotel, restaurant, and attraction rates in whatever form the establishments quoted them -- in local currencies where those were used, and in U.S. dollars (designated by the dollar sign: $) where those were quoted.
Small Change -- When you change money, ask for some small bills or loose change. Petty cash will come in handy for tipping and public transportation. Consider keeping the change separate from your larger bills so that it's readily accessible and you'll be less of a target for theft. Keep a good supply of $1 bills and/or 20,000 VND bills; these will come in handy when paying for cab and motorcycle rides.