In spring, the grape vines are so full of watery sap that a cut from my pocket knife causes it to trickle out fairly fast, and it can be collected in a container. Someone told me that it is a good substitute for water on a trip, because it is purified by the plant. Is this true?
I'll give this a stab, but there aren't any authoritative sources that I've managed to find on the subject!
The most I could really find are examples such as this one where people have drank it and felt no ill effects, and I haven't found a documented case of anyone drinking it and it being harmful to them. From a biological point of view the vine will at least partly purify the ground water, though if it's a known heavily contaminated area I wouldn't take it as a substitute for a proper water filter.
In terms of the effect on the grape vine, most sources I've found seem to show that while it seems alarming it's actually not that bad for the vine, though obviously if you make cuts everywhere and drain all the sap there will come a point where it won't be able to sensibly recover.
In short, if you're really desperate for water and it's the only source available in a survival situation, I'd see no great reason for not giving it a try. It isn't however a direct substitute for a good, purified water source.