I have similar problems with my muscadine grapes. I'm surrounded by row crop farmland, so I'm pretty sure I'm getting 2,4, D drift. I'm told a farmer (or cropduster) can spray it one day when the wind is still or blowing in the right direction and then a day or 2 or 3 later if the atmospheric conditions are right it can rise up and drift a mile or two. I had this last year also. It seemed like it was a problem in the early summer and then they grew out of it later in the summer.
One reason I think it's chemical damage is that I have vines in my back yard in town and they don't suffer from this, but the ones out at the farm do.