SIASCONSET, Mass.— A vandal is at large on Nantucket, an island better known for gray-shingle cottages, boat shoes and billionaires.  In January, someone apparently slashed a 947-foot-long structure of sand-filled tubes, designed to slow erosion of scenic Sconset Bluff. The damage to the project—on which homeowners spent about $18 million, and could cost another $2 million to repair—has inflamed a battle between people trying to save expensive summer homes teetering on the bluff’s edge and year-rounders who say nature should take its course. “It’s shocking,” said Josh Posner , president of the Sconset Beach Preservation Fund. The group is trying to protect…