QUOTE (UglyMan @ Sep 10 2004, 06:13 PM)
Korea has became Vietnam's best friend since the collapse of the Soviet Union. Vietnamese people in Vietnam speak highly about Korea and everything about Korea. I went to Vietnam and I called Korea as 'Dai Han' and Vietnamese people there did not like it. They called Korea as 'Han quoc'. Vietnamese in Vietnam got 'KoreanWashed?" what ya think?
Nam Hàn (South Korea) and Bắc Hàn (North Korea) were commonly used by people who used to live under the government of South Vietnam. When people use Đại Hàn, usually they mean Nam Hàn or South Korea. In few cases, Đại Hàn is also used for the whole Korea.
People who lived under the government of the North VN before 1975 and later the whole Vietnam (called the Socialist Republic of Vietnam), after 04/30/1975 but before the normalization of relationship between the Vietnam and South Korea in 1991 used the words "Cộng Ḥa Dân Chủ Nhân Dân Triều Tiên" (more common, and more official) or "Bắc Triều Tiên" (less formal) for North Korea and "Nam Triều Tiên" for South Korea.
After normalization of relationship, Vietnam kept the same name for North Korea, but changed the name for South Korea to Hàn Quốc as requested by the side of South Korea (I was living in Vietnam at that time. Some people told me the South Korea requested Vietnam's side to use Hàn Quốc as its official name. I am not 100% sure if this is correct). By the way, Hàn Quốc is Đại Hàn Dân Quốc in short.
In general, when regarding Korea as a nation, a history, a culture, Triều Tiên or Cao Ly is used.

Those are official names used for different period of present history. In fact, people can use them interchangeably as long as the listeners understand.
I don't know if the Vietnamese in Vietnam now got Korean-washed. At the time I still lived there, I knew very little about South Korea. Through my friends who are still living there, I know that now South Korean things are very popular.