It depends. If you have an unlocked phone that uses a SIM card, then you can buy a SIM card in Vietnam and swap it for the one in your phone. If have a locked GSM phone, then you’ll have to get it unlocked before you can use it with a Vietnamese SIM card. But if you’re from the U.S., remember that there’s a new law making it illegal to unlock your phone without permission from your service provider. If you have a phone that uses CDMA technology, your phone doesn’t use a SIM card and will not work in Vietnam
You can buy a SIM card in Vietnam at a cell phone shop or at many convenience stores. When I stayed in Hanoi, the front desk agent at my hotel had a bellboy go and buy me a SIM card that came with a bunch of bonus minutes. The cost of the SIM card was only 100,000 VND (around $5 USD). The front desk agent even swapped the SIM for me and set up the phone with the Vietnamese SIM. The SIM was from Viettel, Vietnam’s largest mobile network provider. Call quality was great. The cost of making calls and sending texts was incredibly cheap. I stayed in Vietnam for four weeks and never had to add minutes to the phone. I even called Thailand a couple of times. Other Vietnamese providers are MobiPhone and VinaPhone.