I'm in the process of replacing my shingle roof . The house was originally built with no insulation at the cathedral ceiling. I have damaged plywood that will need replacing so I was planning on pulling all the sheathing off installing some insulation than replacing with new. I've already gotten some good information from previous questions asked on the site concerning ventilation. However, I need to know how to solve the problem of having ventilation from eve to peak on some of the house but because of a long valley part of the roof is only vented that top will this create a problem? If so, Is it possible to ventilate in the parts of the roof that is feasible and insulate the other parts using the methods that doesn't allow air flow. Any suggestions would be greatly appreciated. I've also attached a drawing of the house depicting the long valley.

This is what we have used to insulate an uninsulated log cabin. Eagle Panel Systems: Log Home You can tar paper and shingle right over this and nail in a traditional manner.

If you have to remove the plywood, then you could use any sort of insulating board you're comfortable with putting underneath the ply wood. I would personally suggest stapling thick insulation between the rafters like a new construction home.

For ventilation: they make these plastic things, you can buy in rolls that make a gap between the hipping and the rest of the roof. When you re-plywood the roof leave a 6" gap at the peak and use the plastic venting stuff. The air will flow through the insulation, just not as quickly, thus preserving heat.

Google Image Result for http://www.airvent.com/homeowner/whyVent/whyventImages/house-ridge-multiPitch.gif

Create a baffle from peak to eave. You can use the pink rigid insulation or fiberglass.
You can use two tube like baffles too.

There are many ways to do this, but having a picture would help answer your questions.