If you purchased the iPhone as factory unlocked (available only from the Apple Store in many countries) then it will work in VietNam presuming a cell carrier there provides a suitable voice and data plan. Note that your friend will not be able to get the iPhone serviced in VN should it be required, and would have to send the iPhone back to you or someone else in your country to take or send it for service.
Don't post serial numbers here, by the way. There's nothing we can do with a serial number and it could in some cases lead to abuse by a malicious reader.
I just bought 3 iPhone 4S factory unlock from Apple store. I want to send these iPhones to Vietnam for my family to use but i get nervous because one of my friend bought a iphone 4S factory unlock (acording to apple sale rep) as well from Apple but when he go back to vietnam he found out that the iPhone 4S factory unlock does NOT work because its lock to AT&T. Could it be a mistake by Apple sale rep by selling the wrong iPhone to the customer? Is there a way that i can test a iPhone before i send it back to vietnam? Will iPhone 4S factory unlock work with the local GSM service in Vietnam?