What happens when you go oversea with your iPhone?

Okay, this summer I'll be going to Vietnam and I'm wondering what happens
when your iPhone is oversea. Does it lose all of it contents if you go oversea? Does it still work with the phone being oversea? And if you get another sim from Vietnam does it still have the contents like your previous musics and apps and photos that you've have or downloaded?

Nothing happens to the data in your iPhone. If you are rich, you can even use it in Vietnam! I say that because roaming and especially data-roaming, are EXTREMELY expensive!
To your questions exactly-All your data is still there.
The phone still works, but as I said, it is very expensive.
The sim does not impact your music, photos or apps, so they are all still there.
You can get a vietnamese sim, but you can't use it unless your phone is unlocked. I suggest the following---put your phone on flight mode most of the time. When you are in Wi-Fi areas, like your hotel, internet cafe or restaurant, then turn flight mode off, download e-mails and answer them, send the answers off. When you leave Wi-Fi area, put it back on flight mode. If you really want to be open, then leave it on, but do not use it for calls, making or receiving, unless you have a fortune. You can then be open to SMS sending and receiving, if people get freaked out by your not being available. Even there… Careful… Do not send OK and good-morning messages, as these will cost you the same as a "real" message. We're talking like 1 dollar a pop… In or out. What you can do, is make certain you have Skype and Viber apps installed. Then you can text, call receive calls etc, when-ever you are in Wi-Fi and it costs NOTHING. Just get your friends and family on Viber and/or Skype before you leave. I'm a flight attendant and my boyfriend is a pilot and we travel all over the world, often not on the same aircraft. So we need to be experts in communications without spending all our income on costs. One colleague went to Egypt for a week, did NOT listen to what we told her and with data roaming on, ran up 700€ in ONE WEEK!

If you are willing to pay for it, get a sim card for Vietnam. Otherwise an iphone automatically connects to any networks that work with it, and they are all over the world. When you travel without a sim for your destination, set your iphone as follows:

Settings > General > Cellular
Turn off Celluar data
Turn off Roaming

This way you can still go online when there's Wi-Fi, but it will not inadvertently receive calls or texts etc that are going to cost you a fortune. Everything works and remains just all the same.