This is why we use vapor barriers on the "warm in winter" side of the insulation. People generate moisture. Processes generate moisture. So in the mass of the country, we use a vapor barrier over the insulation on the inside of the house. Even drywall helps, and can serve as a vapor barrier with some care regards penetrations like electrical boxes.
In say south Florida, it is reversed. The vapor barrier goes on the outside of the insulation.
I got a call down there one time to go look at a historic house owned by Dade Co. (Miami). The floors were buckling. I went inside and it was like an icebox. Why? "So we can preserve all the furniture from changes in humidity.". Under the house, the floor sheathing was dripping water.
Ranger asked what he could do. I suggested he unholster his side arm and take some practice shots on all the AC compressors, sell them for scrap, and buy some dehumidifiers.
There was no real way to insulate under a wood house and have an effective vapor barrier at that time. Closed cell foam would work now.
About the shingles, correct, do not insulate the underside of the roof sheathing. This will void the warranty on the shingles and make them die an early death. Insulate at ceiling joist level, and put in a vapor barrier. Ventilate the attic. If the plywood layer is tight, it might do for your vapor barrier.
If you must insulate under the sheathing, use metal roofing or low slope membrane roofing. But at that point, you can put continuous insulation on top of the sheathing and get about twice the bang for the buck.
In this climate, to comply with the Energy Code, I can do R-20 continuous on top of the deck, or R-38 between framing members. Both work out about the same in the calculator.
Note Modesto is in 3B zone. The B is for dry.
http://reca-codes.org/images/ieccmap.jpg