bringing cell phone from USA to Vietnam

Both (iPhone 6 and 6 plus) will work just fine.  If it takes a sim card then everything is all good here.  Just a helpful tip, when they install the sim card, have them turn off the built in ads.  It's not a virus, but it randomly gives you ad pop ups and just gets annoying.

first you would need to have it unlocked then you can use any sim of your choosing.

Stay away from CDMA networks, get a GSM phone. AFAIK, AT&T and T-Mobile use GSM.

Apologies, I just learned that modern phones can handle both standards. My bad.

Cloud9 wrote:

first you would need to have it unlocked then you can use any sim of your choosing.

Right so if your iphone is still under a plan, like if you bought it for about $300 with a 2 year contract, you need to pay off the contract first. And if it is already out of contract, you have to visit your carrier (Verizon or ?) and get them to 'unlock' it in their computer system.
How to unlock your iPhone

Just another note, if you are only 1 or 2 iPhones a laptop, and an tablet with you then there will be no problem with Vietnamese Custom at the airport. But if you have a bunch of brand new electronics with you then there will be a problem since Vietnam Custom do check incoming luggages

This question that you have is exactly what I am now going through; so I will explain everything that you need to know which is quite similar to what others have said before me. I am now in the US and will be returning to Vietnam (where I also live). I purchased a new phone (and number) through AT&T when I arrived back here about 4 months ago. I have had all sorts of problems but with the phone and communication service but let's just focus on your question. I had wanted (and tried) to transfer from AT&t to another service provider but upon discussion with several different ones, I was informed that I must first have the phone "unlocked" which I would need to do anyway if returning to Vietnam. However, here is the sticky point which is that the everyone who attempted to unlock my phone could not because AT&T has a policy that other phone carriers do not which is that in order for them (AT&T) to release their phone to be unlocked, I had to, first, be with them (as a customer) for at least 6 months;  therefore, I must wait until at least October before I can get my phone unlocked in order to use it back in Vietnam. Now, then, my understanding is that AT&T is the only communication carrier that has this policy; all others will easily and happily unlock their phones for use but most of them do not lock their phones from the start, anyway, as AT&T does except that "I Phones" may be a bit different (than the Samsung which I have. Unlocking, otherwise, is an easy process that, depending on who you take it to (a third Party, perhaps) could cost somewhere between $35 and $70. The process can take up to 5 days or be done in one day if physically manipulated but this will cost the most for the service. In response to the other question - No, all new phones do not operate with both systems, in fact, the CDMA system is ONLY used in the USA and a little in China. I know the reason for this but it is not relevant to your question. The GSM system if universally used throughout the whole world and, therefore, for this reason alone is the more preferred system to have your phone operating on. Both systems are NOT available in all new phones.

Check your charger input voltage.
If it's 110 volt only you'll have to buy a new one.

All phones are created now for international use; that's why we have adapters for different power outlets from country to country.