What kind of speeds did you get on these 3G SIMs?
Supposedly Viettel has the largest network but heard some people saying Vinaphone is faster? Maybe more congestion on some Viettel towers?
Also supposedly the airport sellers will sell SIMs registered to others but then they get deactivated after 4 days. So some have to go to the shops operated by the companies to get the SIM re-registered and re-activated.
That said, is there any value in getting a SIM with voice, rather than data-only, which seems to be much cheaper, like 120k VND or less versus 200-300k VND for the same amount of data with voice? That is if you don't speak Vietnamese anyways?