Do i need to jailbreak my iphone 5 to use in vietnam?

Want to take my iPhone away with me while travelling don't know the best way to do it. Do I have to jailbreak it so I can purchase a Vietnamese pay as you go sim card while over there?

No. Jailbreak only gives you access to features that Apple doesn't. Int'l usage isn't one of them. Many carriers offer international travel packages you can add to your existing plan for a defined amount to time. They can also provide you with a sim card from a mutual client carrier partner they have in the region so your phone can run off those towers. Best luck.