I have an AT & T Samsung S4 (locked). Will a sim card for Vietnam work ?
If sim card exchange works will my Contacts be saved?
No other sim will work unless you get the device sim unlocked so you will need to contact AT&T about this. You will only get 2g & 3G in Vietnam and lastly if your contacts are stored on your sim and you use a different one, they will be lost. To transfer to your device Samsung Galaxy S4 - Copy contacts between your SIM and your phone
Once you unlock the Samsung S4, you can swap in any GSM network. Before swapping the simcard, just copy all the contacts from sim to phone so that you do not lose the contacts.