The little bit of reading I've done on grape vines recommends not pruning them at all during the first year. I'd recommend that you let it grow for a year. Then, when it needs to be pruned, you can easily propagate as many as you want. IMO, to prune a brand new plant would set it back and you may be disappointed in the results.... But... I've not grown grapes yet! I have a couple of vines that I stuck in the ground late last fall. One of them is just now starting to show some life.