Options…
• Leave Vietnam and come to live in America. Live in America for 40 days and put the phone on T-Mobile for those 40 days. Then ask T-Mobile for an unlock. Leave America, return to Vietnam. No guarantee without knowing the payment status of this phone if it's been blacklisted or not. If it has, that would mean it needs to be paid off first.
• Find an online third party unlock service. Pay a lot of money, maybe get it unlocked.
• SIM Interposer: Gevey, Turbosim, R-SIM, etc. These are not true unlocks, but will fool your carrier. They will require you to remain on the same firmware - no updates, unless the software/hardware hack itself has been updated first. Some versions may require your phone to be jailbroken.
• Find a phone shop in Vietnam that does unlocks and pay them. More than likely, they'll end up giving you a SIM interposer and doing the work needed to make that function.
• Return the phone. If that's not possible because your 'friend' really isn't a friend, then either sell it or use it as an iPod.
Those are your options. There are no other options, there is no one who is going to be magically able to unlock your phone any other way.
This is why is always pays to be careful when buying US phones outside of the US.