Unless you have arranged a specialist package with your home country cell phone provider then making calls from Vietnam is going to be expensive .The best thing that you can do is buy a local SIM card while you are in Vietnam and then you can use this for marketing local calls e.g. to book tours or to make international calls back home.
There are major phone providers in Vietnam:
All offer reasonably priced packages ,fairly good signal and 3G coverage .In theory before buying a prepaid Sim card you must show your ID or passport however this rarely enforced .
In fact , if you are buying a SIM card from a street vendor you will generally find that the card has already been pre- registered so that you don’t ,need to bother doing it yourself. If doesn’n sound legitimate and they do in fact fake credentials ro register the account ,however it is done for your ease of use and is common practice.
SIM cards vary in price but generally cost $2 and that includes some credit .Event you don’t intend on marking any calls it is worthwhile geting a SIM simply to use the 3G , then you can use translation apps or make calls back home using Wifi based apps
Top tips :Make sure that you bring a phone that is unlocked otherwise you won’t be able to use the Vietnamses SIM card . Locked phones are usually provided when you take out a contract with a specfic cell phone provider.