If you feel like using your iPhone with Mobifone SIM card and you want to access the Internet using Mobifone and not just WiFi networks, you'll have to activate GPRS data access with Mobifone and change the Edge settings on your iPhone.
First, send the SMS message GPRS to +994. You should get a response that says:
Thong tin them: Neu Sim cua ban chua duoc mo dich vu GPRS hay soan tin han voi noi dung DK GPRS va gui den 994de mo dich vu.Then send another SMS message, this time DK GPRS, to +994. I received two follow up messages, both in Vietnamese of course, which I won't transcribe here. Basically you just need to wait a while. For me it started working the next day.
Now to change the settings on the iPhone. Go to the Settings/General/Network/Edge menu. You need to change APN, Username, and Password to the following settings.
Also make sure that Data Roaming is ON. This is under the Settings/General/Network menu.
After that, you should be good to go. I have found the Mobifone network to be pretty reliable. It's great to be able to use the Internet when I'm away from WiFi hotspots.