You should cut the top based on the desired head height of the grafted vine. Make the cut at a clean, unblemished portion of the vine and cut until the height of the rootstock is roughly 4 inches (10 cm) shorter than your final desired head height.
[header] How to graft grape vine [title] Cut the top of the vine. [step] Select a fairly large stock vine and saw off the top roughly 30 minutes before you intend to graft the scions. [substeps] Note that this is the simplest grafting method to use and generally produces good results.