Is the ipSE sim-free an unlocked phone?, can i use it in Viet Nam?
Is Iphone SE sim-free an anlocked phone? Can I use it in Viet Nam?
Are you planing to buy abroad an use the device in Vietnam? If yes, remember that the warranty and service option are only valid in the country of purchase. In case your iPhone needs to be serviced, you'll have to bring it back to that country of purchase. Also, hardware features may not be supported fully or at all in other countries, for example LTE networks.
If so, you should buy your phone in that country where you want to use it permanently.