The largest U.S. outpost of the popular Vietnamese coffee brand Trung Nguyên Legend is open in South Philly. Packer Park residents Estelle Nguyen and husband Vandy Doopened their Trung Nguyên Legend franchise at 113-117 Washington Ave. late last month. The couple transformed a one-story cabinetry showroom into a 5,000-square-foot cafe with two floors and a year-round roof deck, where customers …