First of all,please refer website below.
(The summer edition 2013) to be able to use iPhone5 of Japan in Vietnam | Hanoi Vietnam
http://244room.blogspot.jp/2013/07/iphone5-in-vietnam-2013summer.html*Original was written by Japanese.So,please use the translate function by Google translate.
In Vietnam,it's not so difficult to use mobile for foreigner.Anywhere you can by new SIM CARD by any size.It means normal size SIM,Micro SIM,Nano SIM.Because cutting SIM card is very popular all over ASEAN area.
You can buy SIM CARD here:
1.Airport
2.Telecommunication company office
3.SIM CARD shop.
>Rare number is expensive there.So broker sell and buy many SIM CARD in their shop.
Vietnam's map is quite difficult to read it.I recommend you to connect Internet and use Google map.This is the best way to enjoy your stay there.