Verizon's unlock iPhone 5

Only if there is a CDMA cellular provider in VietNam. According to Verizon there is no CDMA service from them in VietNam. But see here.

You can unlock your phone so you can put any SIM card (that fits) in it. Just call your carrier and tell them you are traveling over seas and they will give you a code that will unlock it fully.

Still the OP needs a CDMA cellular provider. He/she already has an unlocked phone.

BTW, I also read that WiFi is quite prevalent, so you can use that rather than broadband if you wish.

That contradicts to my info, Verizon phone's gsm part is sold unlocked in US.

read first page of that discussion

https://discussions.apple.com/thread/4371703?start=0&tstart=0

"You still do not understand... not certain if it is a language barrier or simply a lack of intelligence.

There are NO unlocked Sprint iPhone 5's available in the US.

The Verizon iPhone 5 does not appear to be carrier locked.

The AT&T iPhone 5 can apparently be unlocked by simply restoring as new in iTunes.

No such option exists for the Sprint iPhone 5 at this time.

Only Sprint can unlock the iPhone and only if you meet their requirements."

and that link

http://www.cultofmac.com/192238/verizon-iphone-5-comes-unlocked-out-of-the-box-w orks-on-gsm-carriers-like-att/