From left, Bui Thi Thuy and Kim Tae-goo and To Thi Vien and Kim Wan-su prepared for weddings in Vietnam and life in South Korea.Credit...Norimitsu Onishi/The New York Times HANOI, Vietnam — It was midnight here in Hanoi, or already 2 a.m. back in Seoul, South Korea. But after a five-hour flight on a recent Sunday, Kim Wan-su was driven straight from the airport to the Lucky Star karaoke bar here,