We picked up a Mobifone sim for 220,000 dong at the Hanoi airport. We probably could have paid less in town but didn't fret over $10-15 purchase. I can't remember the data allotment but I'm pretty sure I chose something that would have allowed us (allowed mobile hotspot tethering) pretty heavy usage over the 3 weeks we were there.
Pretty happy with the coverage as I was able to do a video call over google hangouts while walking up back up from Cat Cat village near Sapa.