Yes Apple Cider Vinegar is okay to put on an open wound.
Apple Cider Vinegar is antibacterial and antiviral as well as being an antiseptic so the Apple Cider Vinegar is a natural remedy for healing and treating infected open wounds.
Wash the area if possible first and then apply the Apple Cider Vinegar to the open wound and let it sit and it will kill the bacteria in the wound and prevent further infection.
You can then after using Apple Cider Vinegar and letting it work apply some petroleum Jelly to the wound to prevent further infection and possible scabs from forming.