One of the first things you need when you arrive in Vietnam is a SIM card for your phone. There are three main carriers in Vietnam: Vinaphone, Mobifone and Viettel.
Many Vietnamese like things free and they will buy more than one smartphone. One phone will have Mobiphone and the other usually Viettel. They talk to the Mobiphone friends on the Mobiphone and the Viettel friends on the Viettel phone to save money.
Viettel has great (and cheap) plans and the best coverage across the country. The service is fast and reliable, even in remote parts.
All have the same quality in cities. Viettel is best in the countryside. So, I recommend Viettel.