After job losses and the mortgage meltdown of 2008, Karin Esterhammer, her husband, and their 8-year-old (autistic) son fled Los Angeles to start over in an unlikely place: a nine-foot-wide back-alley in one of Ho Chi Minh City’s poorest districts, where neighbors unabashedly started into windows, generously shared their barbecued rat, kept cockroaches for luck,…