How and when should I cut back and prune a 40 year old g vine that is severly overgrown?

We bought a home that had had a g vine back in the 60s, our neighbors aunt owned the house, and the trellis it grew on is still there, but trees around the plant have also grown very tall, the lighting over the plant isn't as it once was. The vines have also grown up into the surrounding trees,some 40 feet up, where you can see clusters of gs ripening in summer. The trunk is quite thick with many also thick vines coming off, it thins out, as far as I can tell as the vines grow higher up. We live in North Eastern Pa. Any suggestions?