I believe Gook is used for Koreans and that Dink is used for Vietnamese, but what do I know?
The Korean word for country is gook. Chung Gook is China, Han Gook is Korea (full phrase is Tae Han Min Gook), Mi Gook is America (pronounced mee-gook).
Now, think about this, when the Americans went to help the Koreans, and a bunch of Koreans meet them yelling "Mi-gook!", what do you think the inevitable outcome of that would be?
I've heard Slopes used for the Vietnamese.
I have several friends who were POWs along with McCain and they used the term gooks all the time - never heard them refer to the vietnamese as dinks....