Blackhand, you are stuck viewing things in terms of 21st century black and white ideas of water safety. Those ideas do not apply when vinegar was aded to make water "more potable" Your alleged definition does not even permit something to be better than no precautions.
According to Michael Lee Lanning’s The American Revolution 100: The People, Battles, and Events of the American Revolution, the American soldiers usually received most of their rations, at least in the early days of the war. These included:
1 lb. beef, or 3/4 lb. pork, or 1 lb. salt fish, per day; 1 lb. bread or flour, per day; 3 pints of peas or beans per week, or vegetable equivalent; 1 half pint of rice, one pint of Indian meal, per man, per week; 1 quart of spruce beer or cider per man per day, or nine gallons of molasses, per company of 100 men per week; 3 lbs. of candles to 100 men per week, for guards; 24 lbs. soft, or 8 lbs. hard soap, for 100 men per week.
One unofficial ration was vinegar that made water secured from nearby creeks, rivers, and lakes more potable and added flavor to food. Vinegar’s antiseptic properties also proved beneficial.
"more potable" can only mean safer than before vinegar, as if there were degrees of potability. Further some suggested the vinegar was only to improve flavor however, if that were the case, , the vinegar would not be said to be added to make water more potable but "more palatable."
They believed adding vinegar to water made it "safer" to some degree. We know today, that vinegar does kill some microbes and organisms, even if not all. Even if it would not render the water absolutely "safe" under 21st century standards, there is still some slight scientific evidence supporting their belief that the water became "safer".