Go to Tmobiles website or call cust. care to add a international dialing/roaming package and make sure they have coverage there, then you should be good to go. Unless you are talking about using a SIM card from Vietnam for your US phone, in which case you would have to search google for a "unlock code" pay about 20$ and your phone would then be able to work. Keep in mind if you do have to unlock it, the data side of the phone doesn't always work ie. text mesg. or something. And tmoblie can or doesn't know how to trouble shoot a phone that isn't using there Sim card. Hope this helps!