A prepaid SIM card for 3G is 50,000VND (you will receive plus around 25,000VND). You could find it on any street vendor or small stores in Hanoi. The signal for you to know it is a small table with the sentence "Viettel". To register for 3G, you need to send sms to 191 with content "Mimax" for unlimited data or "Dmax" which costs you 100,000VND but it will give you 1536MB.