HANOI, Vietnam — Phuong leaned closer to a mirror at a booth inside Hanoi’s National Convention Center, dabbing a soft pink Korean lipstick onto her lip as K-pop music blared across the exhibition hall. The 23-year-old student had come for a chance to win a ticket to see Winner, the K-pop boy group serving as the ambassador for the Korea Brand and Entertainment Expo. But like many young Vietnamese visitors moving from booth to booth, she was also there to sample what was on offer, from cosmetics