Posted: 06/06/2017 at 16:30
Just because a cutting has leaves it doesn't mean that it has an effective root system. After all, sprigs of greenery in a vase continue to produce leaves for a while without having roots.
I would leave grape vine cuttings in pots until they have white roots coming out of the hole in the bottom of the pot ... probably 12 months or so ... before potting on.
It sounds to me as if they had insufficient roots to take up enough water to cope with the high temperatures.