I haven't ben to many vineyards with easy access using public transportation in the San Gimignano area...however, I have been to agriturismo Pietrafitta. They have a stand along the main road and I stopped there with my sister. The cool thing was you can bring your lunch or cheese & bread...whatever and they have a picnic place you can sit (snacking on your own stuff) while you do a wine tasting!
Howver you can also visit the cantina (by appointment) and checking it out on the map you should be able to get there by bus.
