We recommend getting a local SIM while travelling in Vietnam; they are affordable, USD $10 per month gets you unlimited data and calls.
There are several carriers to choose from Viettel, Mobiphone and Vinaphone are the most popular. You will need to show your passport in order to purchase a SIM, although there are services at the airport that waive this step. A.R.T recommends Viettel as having the most coverage.
Don’t forget to unlock your phone prior to leaving your home country. Cell phones in general are affordable in Vietnam.