How big is the hole you've seen? Moles are bigger, but voles are smaller, and dig tunnels about 2" diameter. Voles eat the roots of plants and they look like long black mice- they run just as fast when you actually see them.
Here is how I figured out when the voles were active- go out and smooth everything in the area, and cover up any holes, filling them in. Check back every few hours and see if a new hole appears. That way you can tell where they are, and when. Even if it's not voles, you could still try it to see what's happening.
