Cats aren't vole hunters. Dogs are! They have an absolute ball digging for voles. We had a massive vole infestation in our garden last year, so over the winter, we let our Black Lab and Kängal dig out in the garden. They caught (and ate) at least 10 voles, dug up four or five nests, and now our vole problem is gone.
If dogs aren't an option for you, perhaps you should invest in ground-caging. This is a method which requires a lot of work, but it does keep out the voles. Dig a small trench around your daylilies, 8-12 inches deep. Then put chicken wire all around your desired area, preferably as close as possible to the daylilies without damaging the roots, and put the dirt back. You will now have an underground fence to keep them from digging up the roots.
However, since it seems like you already have a big infestation in your garden, I'm not sure even ground-caging will help. That's just a precaution to stop an infestation from happening.
Do you know anyone with dogs you could borrow to go digging in your garden? That is, if you don't already have dogs.
