Vines showing a little heat stress; any thoughts?

Like everything else, my vines have been suffering in this summer's three-figure heat. The leaves on top of the vines are turning brown, some of the grapes have shriveled (above), and a few have fallen off their clusters.

I understand that in the face of weather stress, vines concentrate their resources to support the grapes. So, when I see the grapes deteriorating, I wonder whether this shriveling has to do with the heat or some other problem. Further, all the experts within my earshot say NEVER water the grapes because that will dilute the sugars and, at this time of year, risk the grapes splitting due to sudden excess water.

Any thoughts? Please use the comment feature.