How to use my US bought iPhone 5 in Vietnam?

I'm going to vietnam very soon and I have no knowledge of how I can use my iphone. Will I have to freeze my account or what? Please take me step by step on how I can use my iphone in vietnam.

If its carrier locked you can't use it outside the US unless you ask your carrier to unlock it for you.
If it can accept any sim-card (unlocked) you can just go there and buy a vietnamese sim-card and use it.

Just be careful though when you're calling while walking in Vietnam you're a dream target for pick-pockers

Buy a cheap phone and sim card when you get here. Never leave your stuff in the hotel room or trust the staff at the front desk.
be careful when eating in the cafes in pham ngu lao, saigon. They spit in the food for fun. I know plenty of people doing that. Better to buy street food that the locals eat.

First before you get to vietnam, download viber, tell all of the people you want to contact to download viber so you can actullay contact them, (only with Wi-Fi can you call them or text, its all free) then take out your sim card so that you don't have to pay alot of money for international use, and then bring your phone with you. Thats all you have to do, i'm doing that right now and i'm here in vietnam talking to al my family in america