If you want to make a call to Vietnam from abroad, you have to dial the country code +84.

If you want to call from inside Vietnam, you can go to hotels, post offices or telephone booths. International calls cost about USD 3.
You can purchase a Vietnamese SIM card for your mobile at a favorable price.

You can also phone via internet for free or just a few cents. There are internet cafés in any bigger Vietnamese city.

http://www.worldofinternetcafes.de/Asia/Vietnam