Banks adjust their exchange rates on a daily basis, in line with international money markets, and usually offer the best rates. Large hotels may offer exchange services. Rates are usually less favourable than those offer by banks. DO NOT change money with anyone that approaches you on the streets, it may to be a rip off.
Banking hours in Vietnam are from 8.00 or 8.30 am to 3.30 or 4.00 pm Monday to Friday. Some banks may close at lunch time.
In major cities you can obtain cash in Vietnam Dong (2,000,000 maximum) using a Cirrus, Maestro, Visa and MasterCard at ATM machines. Both HSBC and ANZ Bank have branches in Hanoi and Ho Chi Minh City They also change travellers cheques and give cash advances on credit cards (don't forget to take your passport).
Outside Hanoi, Ho Chi Minh City (Saigon) and other cities you may have difficulty changing travellers cheques and will not find fewer ATM machines. So make sure you have sufficient cash for your journey (either Vietnam Dong or US Dollars). Don't expect to be able to exchange currency in small towns.