Putting a barrier of hardware cloth is a good idea if you have problems with tunnelers and in a backwwards "L" 1 foot deep and 1 foot out can also help some, but voles (field mice) will also come into a garden on the surface and then tunnel down if the want to. Some people have had fairly good success running the barrier up about 2 feet above the soil level, while I have seen these wee buggers climb that high with no problem, just like squirrels will.
The one good things is, like everybody else, voles often will not spend a lot of time and energy trying for food so the more difficult you makes things for them the less apt they will be to try to overcome those barriers as long as a food source is more readily avialable.

Thanks, kimmsr. Kind of figured there was no magic solution. Just today found a local feral cat rescue organization looking for homes for barn cats, so we'll try the buried wire coupled with a pair of cats if we can. The above soil level bit seems awkward to me without bothering with a whole fence. May start and ground level with the "L" and graduate from there. Sure the deer will find us soon enough and we'll need to take the next step!

They've tunneled under the ¼"-inch mesh hardware cloth I buried 6" deep. They've also bitten through the mesh in numerous places.

I'm thinking of digging an 18" trench to install a new fence of heavy-duty mesh. If I bury it that deep, do you think I'll need to form an "L" at the bottom as well ... or will they never try to dig 18 inches down?

kimmsr, have you really seen voles climb a hardware cloth fence that extends 2 feet above soil level? What kind of configuration would absolutely exclude them? I'm sick and tired of the damage they do to my garden.

I have seen voles, and many other rodents, climb many things to reach a food source. As a rule they will not expose themselves to potential predators unnecessarily, but they will also take extraordinary chances if food is scarce.