You can take apple cider vinegar on either an empty stomach or a full stomach.
Some people prefer to drink apple cider vinegar along with a meal or after eating a meal and that's okay too.
However it's best to actually take apple cider vinegar on an empty stomach since it works better to promote your gastrointestinal health and balance your PH levels which helps to encourage growth of friendly bacteria.
So for best results you should take apple cider vinegar on an empty stomach before a meal or before breakfast.