Debra, sorry to hear about your challenge!
A few years ago the vole population exploded here around the farm house, and they infiltrated my seedlings in pots. They can squeeze through small holes, and are happy to widen one for easy access. We were having a challenge with mice at the same time, probably because of bird seed. Had no success with repelling voles with castor oil. I did, however, catch quite a few voles in my live traps for mice. I had tucked the traps along traces that seemed travelled by rodents. The voles would be trapped during the night and would be dead when I found them the next day. Dump and repeat!
We also had several non-venomous snakes nearby, and they really seemed to love a vole snack. Would even enter the voles' burrows to go after them. A natural, country solution. Go snakes!
We don't poison rodents in case it makes it up the food chain. My cat also likes to stalk voles but isn't very effective, unfortunately.
I notice that the voles love tunneling and hiding under mulch. Owls and cats have an easier time catching them when we don't use it, but I use it every other year anyway. I guess the voles have won! I've have more or less given up growing all kinds of favorite bulbs (lilies, saffron, crocus, tulips, iris etc) because the little boogers munch them over the winter.
Frustrating.