I live at about 8500 ft. or possibly a little more. We used to have tons of foxes but they have mysteriously disappeared from our area this past year. Consequently we have a serious problem with voles...more than the neighborhood cats/dogs can handle. I started using a wheat kitty litter because it seems healthier for my cat; I decided to experiment with putting the cat urine clumps in the vole holes of my flower garden. It seems to work...they've abandoned those holes. They have made others nearby so I've decided to fight a war of attrition. Hopefully soon they'll decide to move out completely. I planted mole plant in the vegetable and herb gardens...they have only made minimal attempts to dig there. Any other suggestions?
I rarely see our voles, they're real small and I don't think we have many. We do have chipmunks, ground squirrels, and rabbits. Right now, our rodent populations are really low. I have noticed and read that voles, mice, chipmunks, etc. populations fluctuate with some years there are almost none and some years, they are everywhere - especially the chipmunks. The predator population does the same thing, going away once most of the rodents are eaten and returning after a rodent population explosion.
The kitty litter/cat urine clumps is a good idea. I'm sorry to say I don't have any many other suggestions for you except wait which I guess is not very good. I have used 1 inch plastic gardening netting designed to keep birds off bushes and this works well except than I can't get to my plants and I have to look at them through plastic and move the netting whenever I want to weed or get to the planting area. And if your vole nests are in your gardens already, netting sure won' t help.
I have spoken with other gardeners up here who are having a lot of luck with deer repellent products. I understand that the new repellent products on the market are much better than previous ones. I am thinking of trying some of these next year and maybe polling local gardeners on their opinions on their effectiveness. Maybe you could try one for voles; I've seen vole repellents advertised on the internet.