Pruning grapes at the wrong time
Good day and thank you for using Ask an Expert. Your question is a tough one to answer because it depends upon a number of factors. I have seen relatively young plants either broken or cut back what would be considered excessively and they have grown again. I’ve also heard about old well-established vines that were cut back severely but which survived and thrived.. Even so, there’s no guarantee.
Grape vines are normally pruned during summer to help the grapes size up but, as you know, considerable green growth remains. So, if possible, retain at least some of the greenery, and perhaps rig a temporary support to keep it out of the way during construction. If you must prune hard, understand that the vine may skip fruiting during the next year or so.
Also consider how you might protect the root zone from traffic by people and equipment that will compact the soil. Perhaps a temporary fence? Or a 4-inch deep mulch of coarse bark chips?
After all is complete, allow the vine to recover on its own. Don’t fertilize as even though new green may sprout, it will use up some of the vine’s stored reserves needed to survive the winter and grow again.
“Growing Grapes in Your Home Garden” will likely be of help to you when you must re-train your vine. http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/18935/ec1305.pdf