Vietnamese place names all have meaning. Some are so obscured by time that I can’t determine the origin. But others are fairly easy to define.
“Vietnam” is the name given to the ethnic people who eventually became the Vietnamese. It’s the Vietnamese language version of the Chinese yuëh nan, which means “trouble makers in the south.” That’s how the Chinese termed the non-Chinese people in southern China who eventually moved into the area that is now Vietnam.
The name of the northern capital of Vietnam, Ha Noi, means “river in the inside.” It refers to the body of water around which the city was built in ancient times.
And Hai Phong, the northern seaport, means, literally, “sea defense.” It’s generally understood to mean “coastal defense.”
“Tonkin”—as in the Gulf of Tonkin—is a corruption of the Vietnamese “Dong Kinh” which means “eastern capital.” Related are the meanings of the Chinese cities Beijing (Peiking) and Nanjing (Nanking) which mean, respectively, “northern capital” and “southern capital.”
But not all place names in Vietnam are Vietnamese. The names of the provinces and cities in the highlands along the Laotian and Cambodian border are derived from the tribal languages of the people who populate the area. So Kontum and Pleiku are not Vietnamese names. I have no idea of their meaning.
Thanks for putting up with the meanderings of a linguist’s mind.