I don't think vines need much in the way of manure.We planted two four years ago and each year have had loads of bunches,though never a grape bigger than a thumb-nail.
I never thin the bunches (this may be the cause) and they are all horribly sour.The blackbirds seem to like them though and are very welcome to them.
Last year I placed toy snakes around the vines and the birds left them alone but the grapes were no better tasting for that!
How to manage them properly? A challenge for the Grape-Vine!!