Nobodies really hit the nail on the head.
Philly's oline stinks, Vick doesn't have time to wait for De Sean's patterns to develop. He'll be good vs teams with a poor pass rush and inconsistent/invisible vs those with a good rush. I preferred Maclin because he gets more targets and his routs are underneath.
When watching pre-season games, pay attention to how the olines are playing, although I'd say there are more poor olines than I can ever remember this season.