Ventricular fibrillation fatal?

Usually: Almost always. It may terminate on it's own in some circumstances, but not very often. If you had a internal or external defibrillator, then you could terminate the vf before it was fatal.

Possibly..: In ventricular fibrillation, vfib, the heart is quivering and there is no blood flow so the person is clinically dead. If the heart cannot be restored to a rhythmn where there is blood flow, the patient will then be pronounced dead. Treatment for this rhythmn includes electrically shocking the heart as quickly as possible. Performing CPR in the meantime can be life-saving.

Vf: yes, if it does not stop in time or of if its not stopped by defibrillation in time.