I've written elsewhere on this forum about the damages inflicted on my young trees by voles and chafer grubs.
Depending on my mood of the moment, I've been dealing with the damaged plants either by...
1. dumping them (I have a 'mass grave' where I'm piling the dead / damaged trees for incineration at some later point in time) OR
2. (if they have at least a tiny bit of fibrous roots still left) I cut them back / prune them hard and replant them in a pot, where I can nurse them for a while, till they become suitable for replanting in the open again
I have managed to resuscitate some trees / shrubs such as hazels, hawthorns, willows, and apricot trees. These rescued / resuscitated plants are displaying varying degrees of vigour. Some have already returned to a 'normal' growth and shape. Others are still small.
I'm wondering if anyone has similar experience, and if so, what would you advise? And: do you think it's worthwhile bothering to save the damaged plant? Rather than buying a new one from the nursery?
I would do exactly what you are doing. I will do what it takes to restaure a damaged fruit tree, unless the trunk is severely girdled. Roots can usually grow back. Once this has happened, they will become vigourous. In the beginning you just have to be patience. You will also learn the skills to advise others in similar situations.
It is way to costly to keep going back to the plant nursery to buy new ones. By willing to do nurse your plants back to life, you will likely get the skills needed to propagate new plants too.
You can stimulate those trees to grow new roots simply by watering them with a Vitamin B-12 solution.
B-12 tells the remaining roots to start growing, this is why it is recommended to water in newly planted bare root or even balled trees, it reduces the stress of transplanting and it gets the root system to get growing.
I always tell people to water with a B-12 solution once every two weeks for the first 6 months then once a month for another 6 months so the root system grows fast enough to support the tree.
Those who do this usually tell me that their trees are far more draught tolerant the second year and every year after.
You can make your own B-12 solution with any B-12 tablet, just dissolve 4 per 2 liters then dilute that solution 4:1 to water trees or any plant for that matter.
To minimize damage from the voles, you could put some fine screen around the bottom of your trunks and up to 12" in diameter around the base buried. You could also set out 5 gallon bucket traps to reduce your numbers.
If the trunk is partially girdled, the tree may still survive and be healthy. The wound would heal and the tree would bring up moisture from the healthy side of the tree. But this is a call you must make and sometimes only time will tell if the tree will be healthy or not.
If the vole damage is girdling then you can bridge graft pieces of bark or even pieces of whole branches to the girdled area (I put these about an inch apart when I need to save a girdled tree).
If you need to prevent new damage you can do as Michelle has posted and you can plant double rows of garlic bulbs close together to form a sort of barrier.
Voles don't like the odor of garlic and other bulbs, they won't keep them all out but they will, over time, send out an odor barrier that will turn voles around. (I use this trick for garden beds, fruit trees and am starting to do the entire perimeter)
Keeping any mulch far enough from the trunk also helps keep voles away from the trunks (cats are great at keeping voles away, but our dogs won't let us have cats).