Well, with HYPP, it's a potassium imbalance that tends to set things off the most, and I don't think ACV would be considered an electolyte (Potassium is)...I'm not for sure, I really don't think it would be a problem..I'm sure somebody around here knows more about it than I do. I had a HYPP pos/pos horse years ago..but I never gave him ACV..and they know more about it now.
Can you look up anything on the AQHA site that might mention that..or google the nutritive values of ACV and see what you find. Sorry I'm not much help.
My guess it that it's safe.