Yes Apple Cider Vinegar can kill off the flu virus as long as you take the Apple Cider Vinegar before the onset of the flu.
As soon as you start feeling symptoms of the flu such as body aches, fever, muscle aches, headaches etc then take the Apple Cider Vinegar to help kill off the flu virus before it starts.
If you wait to take the Apple Cider Vinegar before the onset of the flu then the Apple Cider Vinegar won't be able to get rid of the flu.
But if you take it before the onset of the flu as soon as you have the symptoms of the flu then the Apple Cider Vinegar can kill off the flu virus since it's Antibacterial and Antiseptic.
Take the Apple Cider Vinegar with the "mother" the bacteria that is added to the Apple Cider Vinegar for best results.