https://itunes.apple.com/us/app/viber/id382617920?mt=8
https://play.google.com/store/apps/details?id=com.viber.voip&hl=en
I was telling a friend about phones while in VNam and he recommended Viber. He and his group use it while in Ethiopia to call internationally and it has worked great.
Millions of users and I've never heard of it.
It's very good, my goto app. I'd install whatsapp as a back up too (maybe even Zalo).
Vietnam does not participate with Verizon's global phone option. There is also some concern that Verizon dings your bill if it can detect the device with semi-hidden apps. So, an alternate device that is in no way related to your cell provider is the way to go.
Viber has a desktop app. Win, IOS, Linux. I've loaded and quickly tested texting on the Walknbook.
http://www.viber.com/en/products/windows
update:
(something something "ad-infested streams" mumble mumble)
Messaging app Viber adds e-commerce button to sell you items inspired by your chats
https://techcrunch.com/2017/02/23/viber-commerce/