We don't give our grape vines too much water, but on the Sabbatical year when there was no watering at all they produced beautiful grapes and I observed no yellowing of leaves or other symptoms of disease. So I don't think it's lack of water. Overall the local plants (olives, grapes, pomegranates) are very sturdy, suited to the dry climate and don't require a lot of water.
I checked out this table of various deficiencies and their symptoms and it looks as though it might be potassium deficiency but I'm not sure. I mean, why just one vine and not the other? Does this have to do with the variety of the grape? Perhaps adding some compost might help? If someone out there is more experienced in growing grapes and has a bit of advice for us, we'll be grateful!
I do hope it can be fixed - love our grape vines. I love living in a house surrounded by the lush greenery of their leaves. Some time ago, someone suggested I should cut the grapes down if I'm afraid of the wasps they attract, but I don't think I could do that. First, for Jews, cutting down a fruit tree is not a matter to be taken lightly - we would have to consult a rabbi. Second, I don't think I could give them up. They provide such lovely shade, not to mention the most delicious grapes I've ever tasted. So, again, advice is much appreciated.