Is this why the leaves are gone yellow
It could be leaves do have a finite life on the vine and some do die off. The way I am seeing it is that you are asking the plant to support all that top growth through what I suspect is a weak root system. The roots can only supply so much after that limit has been reached it is natural that something has to give.
However there could be a problem with root pests and what you could do, again when the plants are dormant, is dig them out and inspect the roots make sure nothing is munching them. Vines are as tough as boots so you can replant them no problem. I notice the are up against a lawn ... do you ever put any treatments on the lawn? If you do this will affect the vines.
In short, and I repeat, I would dig the vines out in the winter and replant them in a good sized planting hole which has been enriched with some compost and long lasting fertilizers. After the first year of regrowing either cordon train or Guyot prune. Cordon is a good method because you leave a single cane with say seven buds on it wrapped around a wire. These buds will each produce a new cane which will bear a bunch of grapes. At the end of the season you simply prune each of the canes on the cordon back to create a spur with two buds. These should re-grow two new canes in the spring , once they are growing well and the threat of frost is past remove the cane furthest away from the cordon on each spur. Only leave the one furthest away if the one nearest is obviously very weak. This way you will only get seven bunches of grapes but they should be good sized and ripen well. Repeat every year
Here is a good video to follow about planting your vines it is in French but easy to follow the drainage holes are only really required if you have a hard clay subsoil.
Plantation d'une vigne. Raisin de table (variétés résistantes aux maladies). - YouTube