There are 3 big mobile companies in Vietnam.
1.
VITTEL
2.
MobiFone
3.
Vinaphone
VITTEL is managed by Vietnam Armed Forces.On the other hand MobiFone and Vinaphone are managed by VNPT(Vietnam Posts and Telecommunications Group).This group also manage post office.
You can buy SIM CARD everywhere especially Airport or many shops in the city.But in Hanoi(capital city)a little difficult to look for shops.
If you use only calling,any SIM CARDS are suitable for you.The sim card prices and talking fee are both reasonable.That price is start at about 2.5USD(50,000VND).
In addition to you want to use Internet,We recommend MobiFone.Because their plan of internet fee are easy to understand.If you want to use fixed prices plan,you should pay only about 4USD(75,000VND).
Please use efficiently and enjoy your stay!