to score the wall paper so that water will soak behind the paper and the wall area and release.
vinegar does work quite well, but the key is to over saturate the paper. Work in a small square foot area at a time and constantly spraying the outlining areas to actually turn the wallpaper dark with saturated water.
This is not easy and takes alot of patience, but if you saturate properly and score the paper it will release.
I am sure she used the old standby remedy of flour and water to make the paste. It is good but will break down with water.