Before you start pruning grape and blackberry vines you must know and understand it’s fruiting habits.
Blackberry vine pruning. You may want to stake or trellis-train your berry plants to keep them more compact and upright.
Pruning will vary depending on the blackberry variety you plant. Most berry bushes bear only once on 2-year-old canes. After the canes have produced fruit, you should prune them back to the ground to leave room for the stronger, 1-year-old canes.
Some pruning should be done every spring to keep the plants from becoming tangled and to improve their ability to bear. Prune trailing blackberries in the spring for good growth habits. Prune each main cane back to 3-4’. Then cut back side branches to about 12”, leaving five or six buds on each. Erect and semi-erect varieties should be tipped or cut back to 3-4’ in midsummer. This forces lateral branches to emerge from buds below this point.
Later in the fall, after they are dormant, cut back the laterals to 16-18”. Fruit will be borne on these laterals the following summer (after which, the canes should then be removed to make room for next season’s growth).
Everbearers fruit twice on the same cane. These canes will fruit at the tip during the fall and then bear again the following spring farther down the canes. If one large crop is desired, cut the canes back to the ground after the fall crop. This will result in a single, large crop the following fall.
Grapevines pruning. Grapevines need weeding, fertilizing, insect and disease control, and proper pruning to assure a bountiful harvest. Proper training of grapevines is essential to maintain plant size, shape and productivity. If left unattended, grapevines can become unruly, and fruiting will be poor due to overproduction of vegetation.
You must be ruthless when pruning grapevines to remove 80 or 90 percent of last years vines to ready what remains to produce this years crop.
I think this video can help you understand grapevine pruning much better than I can describe here.