I will be traveling all over Vietnam and I would like to be as often connected as possible.
So having a sim card with the mobile network having the best 3G coverage is key to this aim.
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There are mainly 3 mobile network operators in Vietnam. Viettel is currently the biggest one and also the only one that invests in other countries (Cambodia, Laos, Haiti, Peru...). It has very good coverage in rural areas. In remote islands or highland areas the only operator is Viettel. Reason for those? It's operated by the Ministry of Defence so they must increase coverage as much as possible even if there's no profit in those areas to improve national security and defense. In urban areas most operators will be fine.
The next 2 big network operators are Mobifone and Vinaphone. They allow voice roaming to each other when the main one has no signal, so the coverage is also good, but I'm not sure about data roaming.
There are 2 more small operators: Gmobile and Vietnamobile but the coverage is generally not good even in urban areas. Gmobile is 2G only and Vietnamobile only supports 3G in a few large cities.
There are 4 major network operators in Vietnam.
The three largest - Viettel, Mobifone and Vinaphone are all government owned, and right now all have very similar prices. You're looking at about 120,000VND (about $6) for 1.5GB of data which will expire after 30 days.
Vietnamobile is a smaller operator, part owned by Hutchison (who own the various 3 networks across Europe and Asia). They're cheaper, at about 40,000 VND ($2) for 2GB of data, which also lasts 30 days.
So why would you pick one of the other operators? Vietnamobile's coverage can be patchier, although it improved significantly in early 2017 to the extent that on a recent trip from Hoi An to Phong Nha Ke Bang (a small but slightly touristy town) I had 3G signal virtually all the way. Still, off the major highways and outside towns, the other operators do have better coverage.
It can also be a pain to find a Vietnamobile scratch card for topups - I wasn't able to find one in the international airport in Hanoi last time I looked, although about half of street vendors in Hanoi seem to have one hiding somewhere. SIM cards are harder to come across. So it depends on whether you think the cheap data is worth the short term hassle.
FWIW - I use Vietnamobile, but I'm going to be here a while.
Just arrived in Hanoi. Bought a Mobiphone card. Was unable to data share towards a second phone with an USA number.
A Vietnamese local advised that her phone with Vinaphone network does share data.
Went back to the company at the airport that sold the Mobiphone card. Correct... You can not share data with a Mobiphone card. Bought a second card from Vinaphone within 30 min after the first purchase. Problem solved.
If you want to share data from a vietnamese sim card. Then do not buy Mobiphone! Buy vinaphone.