Developers are proposing to start construction on more than 1,500 apartments on the South Boston waterfront in coming months, hoping to replace some of its vast parking lots with sleek glass buildings that will lure more residents, retailers, and shoppers. The projects range from a pair of towers across from the federal courthouse, to a 21-story apartment and retail building at Pier 4, to a 197-unit apartment complex on D Street next to the Boston Convention & Exhibition Center. Together, those and many other developments promise to provide what the waterfront lacks: A sense of vibrancy on streets that currently empty out after 5 p.m., when thousands of office workers head home for the night.