Gaia Borino decided to start her 10-day American vacation in Covington, Ga., for one simple reason: “Vampire Diaries,” she said. The 22-year-old Italian, who lives in Luzzane, a town of 600 about 30 miles outside of Milan, landed at Hartsfield-Jackson Atlanta International Airport on Dec. 24 but immediately ran into the kind of trouble that discourages many people from traveling in the first place: her car rental reservation fell through because of credit card issues and she spoke little English. After giving up on the rental, Gaia (pronounced GAY-ya) took a $110 cab ride to Alcovy Road,