Compared to that of neighboring countries such as China and almost all the countries in the world, mobile telephoning and mobile Internet in Vietnam are indeed very cheap. Just buy a pre-paid card in the streets and you will be able to call to your home countries for as cheap as 4,000 VND per minute (around 0.19 USD or 0.15 Eur as of Aujust 2013). Domestic calls to land line and mobile numbers from a pre-paid simcard cost around 1,300 VND per minute. Also the good thing is, you often get a call credit as much as 2-3 times the price you pay for the pre-paid simcard. For instance, if you buy a simcard for VND 50,000, you can have VND 100,000 to VND 150,000 pre-paid call credit. Operators also have special offers as often as 3 times per month so if you buy scratch cards to recharge your account during those special offer days, you will get 50% more of the call credit (recharge VND 100,000, get VND 150,000 in your account).
Stick with one of the 3 major operators which are Mobilefone, Vinaphone and Viettel. They are the prevailing GSM operators in Vietnam and offer good quality service and also their mobile Internet service is reliable. In Vietnam, mobile Internet is very convenient. The 3 mentioned prevailing service providers offer 3G (HSDPA) service in all over the country for as cheap as VND 50,000 (around USD 2.5) for 1 GB data download and upload. You have to register on you phone to use that special package rate, otherwise the rate will be higher. If you register for that package, the good thing is they allow you to continue using data service at 256 kbps at no additional cost after 1 GB is used up.
The current law if Vietnam requires that pre-paid mobile phone users have to register their identity before the simcards can be activated, but don't worry. Shops often sell simcards that are already activated. The simcards are new and never used, but the shops already register the simcards under someone's name (fake names) so that they don't have to register or take any actions at the time they sell the simcards to you.