How many fluid ounces of vinegar should be mixed with 80 fluid ounces of water

That depends on what you are making.  If you are going for the vinegar cleanse thing that some people do on their face, then I would recommend either using 120 oz of vinegar or 160 oz vinegar.  If you want salad dressing, then you don't use water at all, and instead you use olive oil and salt, Dijon mustard, pepper, and something like chopped shallots, and with this you would use only 1 fluid ounce of vinegar (this is a recipe you can find on epicurious.com).  So it really depends on what you are trying to do.  I personally like my water without vinegar, and I think that vinaigrette salad dressing is a lot better without 80 oz of water, but that is a personal preference.