I doubt the culprit was a garter snake. More likely a cat, well fed, from the nearby barn. Our cats out here on the farm routinely kill Voles and leave them on the sidewalk at the bottom of the stairs coming off the decks. Voles covered in saliva may actually be covered in dew. There are other things that kill Voles but most of those critters (coyotes and bobcats among others) will eat them on the spot. I have heard that horses will kill them by biting them but have never seen one do it. Bull snakes are also potential culprits and a large one can handle a Vole easily. May have been spooked off or was just too full to eat it.
Nice post and good photos. Thanks.