Hi Eric,
I worked in Saigon as a programmer for the last 4 months. Internet is generally good, and 3G serves as a fine backup. Most cafes have enough bandwith for voice calls over Skype, WhatsApp or FaceTime. I must say that FaceTime consistently outperforms the other two in terms of quality and connectivity.
The better workplaces (Work Saigon and Dreamplex) usually also have a good enough connection for uninterrupted video, where again, FaceTime was consistently better than Skype, Hangouts or Appear.in.
I practically never had problems with audio over 3G.
There are stories about โthe sharks biting the cableโ every once in a year, slowing down all internet down to a halt for a week, but I havenโt experienced that fortunately.