On your situation I presume you will be putting up a vapor barrier then filling in behind it with the blown in cellulose. The vapor barrier always goes toward the heated space, so that would be right.
The ventilation needed is a channel from the soffit vents to the ridge vents. You can buy formed plastic channels and staple them directly to the underside of the roof. Each space between rafters has to be vented in this way to prevent moisture buildup.
Take into consideration the size of your rafters and the space taken up by the vent channel. Your r-value will depend upon the thickness of the insulation and the vapor barrier.
What will you use for vapor barrier? I don't know if plastic sheeting will support the weight of the insulation especially when it's hot. House wrap might work but I donít know if it acts as a moisture barrier. I'm sure that someone here will have a suggestion.
I recently had an addition built with cathedral ceilings. The contractor put the plastic channel 1/3 of the way up the ceiling, insulated and covered with dry wall. His dry wall job was the worst I have ever seen. I couldn't let this go so I removed and replaced most of the drywall.
When I opened the ceiling space I found mold growing on the roof and rafters. The fiberglass insulation had prevented the airflow to remove moisture. This was after only a couple of months and the space wasn't heated yet. I had to kill the mold with ammonia solution, remove all the insulation, and install the plastic channels all the way to the peak. The rafters here were 2x6 with a span of 8 feet. The channels take up between 1.5 and 2 inches of space.