Is it Vietnam or Viet Nam?

Once upon a time I learned that my country was called “Vietnam” in English, and it was a thing throughout my school day. As I graduated and got a job, it became “Viet Nam” all of a sudden. Now every textbook I see is all “Viet Nam”, as if “Vietnam” has never existed. What happened? Did I just somehow teleport to a parallel universe one day and now I’m the only one with a non-existent knowledge? :D

What does it called in English by actually English speakers?