Probably depends on which coast youre in. West coast and California is probably Vietnamese and east coast and new York is Korean.
Around D.C. it's a mix (same with Barbers)
It helps to have the 3rd highest concentration of Koreans in the U.S. and a large Vietnamese population in Northern Virginia. Go down one street and there's nothing but Vietnamese nail and hair businesses. Go down another and you'll find Koreans.
http://www.bbc.com/news/magazine-32544343