Originally Posted by
madbrain
The prepaid cards are not expensive. The problems are:
1) requirement to unlock both our phones. You can't use the VN SIM card otherwise. This is a process we have to repeat every time we upgrade phone.
2) It makes it impossible for us to take advantage of any T-mobile phone upgrade plan, since they won't give the unlock code unless the phone is paid in full.
I've found it's been easiest to just buy an unlocked phone, either as your main one (e.g. one of the nicer Nexus or OnePlus devices) or as a separate travel phone (the cheaper midrange phones like the Moto G or Honor 5X are great for this.) I've done both, and prefer having a second cheap phone for travel; means, among other things that I am not going to lose my expensive one (with data that might be of concern) in a crowd.
Not as useful if you need to be reachable on your US number, of course.
3) it wastes time on a trip to go hunt for those SIM cards on your first day.
I haven't been to Vietnam, but in a lot of other countries there are SIM card booths at the airport, and/or convenience stores at the airport which will sell a SIM card there -- and the level of English tends to be much better than once you go further afield.
There certainly are exceptions -- Japan is a very unfriendly place to get a SIM card as a foreigner! -- but the "SIM card at the airport" seems to be a very common case (although sometimes pricier than outside, e.g. LHR where I think I paid GB#30 a few years ago for a SIM and credit that would have been half that off-airport)