The prognosis depends on the circumstances leading to the vfib. If it is a response to a sudden 100% blockage in the left anterior descending coronary artery, that is a widowmaker. No amount of defib will "fix" that until the blockage is cleared, and there is only a window of a very few minutes to clear it by going in surgically, or a hail mary measure to dissolve it fast.

If the vfib is "just" a result of chemical imbalance or a less major blockage, the outlook with vfib is much better, and it has even been known to spontaneously self-clear.