Unlocked iPhone 3 or 4

It means that a cell/handset is in a state to accept any SIM.

Carriers offer handsets at cheap or zero cost but they 'lock' them so only their network will work with it.

Ithingies need 'jail breaking' to use applications unapproved by Apple. This involves replacing the Operating System with a modified one.

I use Android so I am free to do a lot of things that Ithingies can't.

SEE ALSO: < https://www.expat.com/forum/viewtopic.php?id=175761 >

If I bring any iPhone 3 or 4 from the US, can someone unlock it in VN?  Is it very expensive?  Is it better if I bring an unlocked iPhone?

Sure, VNPT and Viettel can both unlock it, legally, if you subscribe to their service.

Other outfits might jailbreak the Ithingy as a way to unlock it, so make your wishes clear.

If you are coming for only a short break, you can buy a cheapy smartphone for around $30 - made in China but with English menu.