I'm curious what part of the world you live. I had no idea what a Vole
was until a few years ago, when they nearly overtook a town called
Folsom, California. They are still a problem there.
They're very cute, but very destructive.
I worked for a company that had them on-site in El Dorado Hills - not
far away! The building was at the base of the Sierra Nevada foothills,
which had all sorts of wildlife. . .Deer, Mountain Lions, 5 1/2 foot
Rattlesnakes, and especially Voles.
We had a reasonably good pest control company, and they set up little
traps near entrances to the building. You know the type, they go in,
but they don't come out. There was some sort of bait within the
housing of the traps.
I'm wondering if you could put some type of bait down those holes and
tunnels you are finding around your place? That's if you don't have
pets in your yard. A safer bet might be those type of traps my company
used. I've seen them at Home Depot.