I would prob choose Viettel over Mobi. When I first arrived in vn and used the phone a **** load and so loaded it numerous times. Each time you load it, you increase the date where u can still use the phone and not load more money. In other words, the phone would work for incoming calls if you have no money in it. Im good till 2016. I even went back to the states for 8 months, came back and it still worked. That varies as I hear from people that if a sim isnt used at all for a month it will be disconnected. Idk, its what i hear.
I now only use like 20k a month so even though I want to change to Viettel, I rather just keep the free phone usuage.
Promotion wise, Viettel give the 50% promotion more often than Mobi does and Viettel still gives a straight 50% while Mobi does also give 50%, they split it into 25% local and 25% long distance which I dont see as a problem for most people.
Every sim card in a phone is able to use 3g. If you want one to use for a computer, here are the links. You can go into any retailer, most of them have info for 3g.
http://www.3gviettel.com/
I have this and I like it. I paid alot for it when it was new. Now according to the sight, its only 65k to buy the usb stick. If you buy this you need a vn id number of someones. I dont use it for anything heavy. I think its cheap to use.