Hello, My wife will be travelling to Vietnam for 1 week for work. She has an unlocked Verizon iPhone 4S. What would be the easiest way for her to purchase a sim with data and voice? Preferably purchasing at the airport on arrival would be best. Obviously, she will be tired on landing so looking for something easy and fast. Location? Brand? Cost? She will be staying at the downtown Sheraton Saigon hotel and towers so getting a sim within walking distance of there would be good also. Thoughts?