In this publication on grapes it says that it is best to prune neglected vines in stages.  If you work in stages you will eventually get better canes and fruit. It would take a few years to accomplish that. You could also cut it back with the understanding that it may or may not work and it might take a couple years to determine that. You could consider replacing it with a new one and plan to do a better job of pruning right away. Here is the link I was talking about, you will have to scroll down to the area where it talks about pruning neglected vines. Good luck!
http://www.extension.umn.edu/distribution/horticulture/DG1103.html