Vines shouldn´t be cut back before winter because the remaining sprouts might freeze depending on the intensity of the winter. If you leave the branches as they are they can still freeze at their tips, but this can be corrected in the next season. You will notice that this year's branches will develop side shoots next year which will then bear fruit. So keep as many of this year's shoots and cut them down in late winter to 5 to 7 buds. Make sure not to cut vines too late in winter, otherwise they might “bleed”.