I'm currently using Viettel. At least in their main shop here in Nha Trang the staff spoke good enough English that they were able to understand what I wanted.

I'm currently using Viettel. At least in their main shop here in Nha Trang the staff spoke good enough English that they were able to understand what I wanted. Just tell them you want the 3g plan with the monthly fee. You may have to go into an actual shop to get this set up. The people selling sims on the side of the road aren't very tech savvy in my experience.