When I was in Vietnam, I had to make a lot of phone calls when travelling to different places. And I encountered many difficulties since I did not have a mobile phone. Then my nephew who was working for Eviva Tour Vietnam showed me how to get a mobile phone in Vietnam.
You need to know that the mobile phone service in Vietnam is very different from that in Western developed countries. In Germany, for instance, you sign a contract with a network provider first and then get a mobile phone with a connected SIM card inside. If you do not terminate the deal, the network company will automatically renew the contract when expired and provide you new phones depending on particular situations. However, you do not need any contract in Vietnam. There are two separated things you need to have, a SIM card and a mobile phone. Both of these are very easy to buy, especially in big cities like Ha Noi, HCMC, Da Nang, etc.
My nephew told me that I could find a Sim card in any store which sells things such as newspaper, books, stationery, small stuffs, even food and drink. Currently, there are three major mobile network providers in Vietnam: Vinaphone, Mobiphone and Viettel. I chose Viettel which was recommended as the strongest one in the country at VND50,000 for one SIM card. You do not need to worry about the price since it is fixed and clearly shown on a board hanging on the wall of every store.
Buying a new mobile phone even is much easier. Just go to any commercial shopping mall, or more carefully, take a search in prior on the internet and then go buy the phone you like. The price range is from VND500,000 for simple models to VND10,000,000 for advanced ones. Once you have a mobile phone, just follow instructions on the SIM card to get your phone connected. Or simply ask the phone seller to do it for you. After a few minutes, you are now able to call anyone you know in Vietnam.