Voles are more like mice.They will travel in mole runs, but they are a differant critter.Still, don't spray the trunk, that does stress a tree, just let the water soak in around the roots.
I did not mean to suggest overfeeding, I just think it's best not to fertilize when a tree is stressed like you describe.It sounded like root damage at first and may be true the root ball wasn't big enough to begin with.
If you keep it watered, it may pull through.Best of luck.
Thanks so much for your advice. I am a "newbee" at tree care. I am 67 and always just stuck the tree in the ground, walked away and it grew--no problem. Now that I have time to look and worry all this bad stuff happens.