You didn't mention whih species you are breeding, however I have noticed that in labrusca seedlings (they grow wild here), the thicker the cambium layer, the better the yield. Short internodes are better. Late budbreaking is better, but not too late. Around the same time as when apples are blooming seems to be best.

Vigor isn't necessarily a good thing, however I have noticed that the plants that are sturdy enough to be able to support themselves somewhat do much better than the ones that are extremely flexible. What I mean is: the plant may grow straight up or at a slight angle for the first 2 ft or so before curving downward under its own weight. The wild ones growing like his will hit the ground several feet away and take root. They do MUCH better, survive better, make better fruit, etc., than the ones that trail like a limp rope.

I haven't seen much correlation between leaf size and berry size.

Good luck.

I agree that leaf size and berry size do not correspond. My largest leaf is on vitis Coignetiae, which can have tiny fruit and clusters as a species, though I've found a hybrid 'I think' of V. Coignetiae with clusters about like Leon Millot, and they taste pretty good, so Ive grown out a few seedlings of that one.