Hi yellowtriumph,
I'll chip in with my 10 ¢ (2,300 VND) worth. I lived in Saigon for a total of 11 years - 3 in the 90s and 8 from 2007.
In the 90s, US dollars were commonly used for payment in Saigon and in fact you could get them from ATMs! Nowadays, paying in US dollars is very rare and it has been actively discouraged by the government. I would pay for everything in VND - that way you don't have to worry about being ripped off with the exchange rate or falling afoul of scams where they substitute a forged US bill for the one you handed over - a friend of mine lost $100 that way!
I would bring a small amount of US cash - one hundred dollar bills for currency exchange and then small denominations for use in Cambodia, as others have advised. Make sure all the bills are in pristine condition - dirty or creased ones are often rejected. In the big cities (Saigon, Hanoi and most others), foreign credit cards and debit cards can be used at almost all ATMs to obtain VND and in my opinion, that is the safest way to obtain local currency. If you go to foreign bank ATMs (Citibank, ANZ, etc.), you can take out larger amounts - up to 8 million VND ( ~USD 350) with a fee of a couple of dollars. Local banks usually limit you to smaller amounts - 1 to 2 million.
I would not get VND outside of Vietnam. The rate will probably not be very good. If you arrive and need local currency at the airport for a taxi, then change a small amount but wait until you are in the city if you want to change larger amounts, although as I said above, ATMs are the best way to obtain VND *. If you want to change some currency, then don't do it at your hotel - go to a money changer. If you are in Saigon, I can recommend the money changer on the corner of Dong Khoi and Nguyen Thiep streets - right in the middle of the District 1 tourist area and across the road from the Sheraton Hotel. Very reliable and good rates.
A note of caution - when using an ATM, you will often be given the option of converting from VND to your bank account's currency at the terminal. I don't recommend this - you will usually get a better rate letting your bank do the FX conversion.
I would only use credit cards in hotels and possibly in tourist oriented restaurants and bars, but again, I recommend using cash. Many places charge a 3% commission on credit card transactions.
Other advice. Vietnam is a very safe place to visit, but just be aware of your surroundings. Don't stand on the edge of the pavement, holding up your 15 million VND phone. A passing motorbike may relieve you of it! I back up to a shop window if I want to use my phone in the street. The same risk applies to gold jewelry and fancy watches - leave those at home.
Local SIM cards are very cheap and include Internet access, and are a good way to keep in touch with other members of your group. Almost all bars and restaurants have free WiFi. Your hotel should be able to help you get one or direct you where to go to get one.
If you haven't got a visa yet, I recommend doing it online, since you don't have to send your passport away. For a small fee, the online company sends you a Visa on Arrival Letter, and you then obtain your visa when you arrive at the airport. For a small extra fee, you can get personalized service which allows you to jump the queue. Recommended. I have received very good service from
www.evisa.com.vn.
I can pass on more advice if you let me know where you are planning to go.
* I had a friend visit me from Canada and he asked me how much he should get out of the ATM. I said "At least a million, preferably two." He said "I don't feel comfortable taking two million out", but the next day he admitted that I was right, because he needed more money. He took his ATM receipt home to show to friends - he had used his long term savings account and the balance was 83 billion VND!