At the bustling Han Market in central Da Nang, two Vietnamese vendors tease a middle-aged South Korean tourist by calling him harabeoji, or “grandfather” in Korean. “I’m offended!” he jokes. The women grab his arms and laugh. “It’s OK, it’s OK.” The man had returned to their stall after buying 200,000 dong (US$7.60) worth of macadamia nuts the day before. It is a small moment of banter, repeated dozens of times a day in a city that many people from South Korea jokingly call their own unofficial...