Vacant property may not be a sexy topic, but it sure is hot. A convergence of gatherings in the city last week attracted about 470 people to vacant-property sessions, in part owing to the conference of the National Trust for Historic Preservation, which wrapped up a weeklong visit yesterday. Most of the crowd assembled at the Pittsburgh Project on the North Side for a first-time, two-day Pittsburgh Green Forum Thursday and Friday. The challenge was: What actions can change vacant land from liabilities to assets for neighborhoods?