Voles do eat plant roots which can kill the plant. Dig up your Hydrangea to see if it still ahs a good root system or not.
Castor Oil, sprayed on your lawn, has been used for many years to deter moles. The Castor Oil is reported to coat the insects the moles eat making them distasteful to the moles. Since the moles do not like the taste they leave to find better flavored food. Since Voles are vegetarians
that probably does not work on them, although you might think the Castor Oil would still coat plant roots.
The only reason to not spray Castor Oil if temperatures exceeed 80 degrees is if you are covering a shrubs leaves.

If your plant is 'just dead branches' and has no leafy green growth, then yes, it is most likely dead. Remove it and replace it. The only effective precaution against voles is caging the root system - wrapping it loosely with chicken wire. Works not too bad on young, smaller plants - larger plants generally do not require as much protection.

I would find it highly unusual for a 15 year old hydrangea to be killed outright by voles. I'd suspect some other, probably pathogenic, cause. Digging up the plant since it is dead already and taking the rootball (or healthy portion of it) into best local nursery for diagnostic help or to your county extension office is a good start.

gardengal - Guess what, I could not believe it - I pulled slightly at the hydranga and it came right out of the ground with no problem. There were no roots at all.

How do you cage the plant with chicken wire - do you put it in the hole you dig for planting and place the plant in the middle? Sounds like a good idea but how far down does the chicken wire have to go?

Thanks.

Chicken wire has a mesh of 1 or 2 inches and it will not be an effective deterent for a mole which can squeeze itself through much smaller openings. Hardware cloth with a 1/4 inch mesh probably would be a better choice.