After petitioning federal labor officials to let them have union elections last month, full-time and salaried faculty at Boston University voted in favor of unionizing by 4-to-1 margin Wednesday. The vote allows them to join more than 3,500 Boston-area educators who are part of Faculty Forward – a division of Service Employees International Union Local 509. “This is tremendous day for faculty, our students and the entire Boston University community,” Bill Marx, a senior lecturer in the College of Arts and Sciences’ Writing Program said in a statement. “Today’s vote gives full-time and salaried lecturers and instructors the proactive voice we need to improve the teaching and learning conditions on campus.”