The grape vines in vineyards are taken back hard each year. If you've got a climbing one that is partly to look pretty, you might want to be a bit more lenient with it. We had one climbing along our balcony in our old house and that got trained much like climbing roses or something so it had the central stem and then offshoots got pruned back to that each year so it retained the overall length and covered the railings nicely. Definitely prune in winter when it's dormant (although removing leaves to allow the fruit to ripen is fine),
H