I know they could have performed better, but the Falcons' defense is not that bad. Plus, it hurts them alot when they don't have an offense that can put together time-consuming drives, and to get better field position (which relies on the QB position). Its hard to hold a team down to low scores when the offense throws an interception that gets ran back to within the 20, or fumbles in their own territory. Also, when they are tired from coming off the field and forcing a punt, and then after a 1:30 minute drive they have to go back in, it doesn't do anything to give them support.
You need stability and control at the QB position, which Vick has none of.