Also, I think its worth emphasizing that you do NOT want to ride a bicycle on Highway 12 between Fairfield and Lodi (Rio Vista is about halfway between). There are narrow to no shoulders, heavy fast moving traffic, including many big rigs, often reduced visibility from tule fog, and a nasty accident rate. This is one of the most dangerous roads I know of on which to ride a bicycle.
I agree that Montezuma Hill Road is nice. The best way to get to it is to ride on the bike path on the north side of Hwy 12 to the east end of Suisun City. The path begins near the pedestrian overpass between Fairfield and Suisun. When that gives out, you'll have to ride on 12 for a couple of miles (this is the least dangerous part of Hwy 12). Turn left on Branscomb road (and be careful crossing the highway), then right on Creed Road, and right again on Denverton Road, which will take you back to Hwy 12. You'll cross Hwy 12 to continue on Denverton Road for about a mile before you are returned again to Hwy 12. Then you'll have to ride on 12 for about a quarter mile before you can turn right onto Shiloh road. After a while, turn left onto Bird's Landing Road, then right on to Montezuma Hill Road, which will take you all the way to Rio Visto.
Having never ridden past Rio Vista, I'm not sure what the best way is from there, but it isn't Hwy 12. Probably north from Rio Vista up the west side of the river to the ferry to Ryer Island, and then route through Walnut Grove, Thornton, and then across the central valley between Galt and Lodi towards Ione.