The answer is very simple. Just buy a local Sim card!!!
Where to buy a Sim card in Vietnam
It is very easy to buy a Sim card in Vietnam as you can find it at nearly every shop at a very reasonable price: VND 80.000 (about 4 US Dollars). Once purchasing one, you can press the sequence *101# to check the info on credit and expiration date. Normally, you will receive a message shown a bonus of at least 20% for making calls and texting. International calls are pretty cheap, around VND 4.000 per minute fora direct dial. A Sim card normally excludes data and you have to register by sending an SMS to the wireless provider. However, this is not a big problem as you can connect to the internet through free Wifi which is available at every hotel, restaurant or shop.
Kind of Sim card
If you use all the credit in the Sim card, you can buy the scratch card at any shop to extend. The card costs VND 50.000, VND 100.000, VND 200.000, and VND 500.000.
Among many Sim cards sold in Vietnam, you should choose Mobi Fone and Viettel – the biggest wireless providers at the moment as they won’t spam you with SMS every hour.
To be suitable for the local Sim card, you should also bring an unlocked phone GSM compatible and be swapping Sim Cards. An old spare mobile is always preferable even it can’t help you to connect to the internet.