How do I install roofing material on a roof with a ridge vent?
You will need to remove the ridge vent so you can shingle up to the cut opening, and under where the ridge vent attaches.  If you can salvage it great, but otherwise, install a new ridge vent. Do not try to mickey mouse your last row of shingles under it to avoid taking it off. You WILL be sorry the first time it rains.