I have an old (10+ years) grape vine, which has not been pruned in at least six to eight years. It produces, but of course its several branches are very long. Would I be wise to prune it back severely, to the first branching of the vine for best future health and production? I'm assuming that would certainly lose this year's grapes, which is okay, if we can grow larger, healthier grapes on a well-tended vine in the future.
It is best to prune back the vine to the form that you want. Since it has not been pruned for years, it will require you to prune back to older wood, and you will not have fruit this coming year. Keep in mind that only 1-year old wood (shoots that grew last year) will contain fruitful buds. It would be best to prune back your vine to the trunk with a few spurs (if possible) to grow during this summer. Try to form the vine to your desired shape (on a trellis, hopefully) during the summer, and remove extra shoots. Next winter, you should plan to prune back your vines and maintain buds on those 1-year old shoots for next year's fruiting.
If you have future questions regarding your vines, please contact your local county Extension office and Master Gardener program for support. Also, please see our Extension catalog for helpful documents and guides from OSU Extension Service.