TE:

It won't hurt the plants but will knock out your harvest for 2013. And it won't solve your long term issue which is learning how to properly prune and doing it every year. Pruning grapes is easier than it sounds and something you will enjoy once you understand how it's done.

If you instead try to regain control and correct this issue over two years instead of one, you won't miss a harvest completely. Next spring prune off as much of the old wood as possible while still leaving a few canes with this summers wood at the ends of the shoots. Then when you prune in spring 2014 you will be have lots of new growth and can prune properly.

You will need to remove 80-90% of the canes as short as possible next spring in order to really turn things around.