You would be suprised at how small the opening can be and a rodent can squeeze in it - seriously less than an inch of an opening and they can get in. You need to check all of the walls, cabinets, closets, pantrys, door ways, basements, attics, etc for cracks and openings caued by wear, mildew, or the house being old. You can fill them up with caulk or some other type of filler and solve the problem of them getting in. I feel ya on this one - we had an opposum get into our house from a hole that had been caused by mildew under our cabinets that we never used...it wasnt fun. Make sure to set traps where there have been signs of them staying around also to be sure there are none still in the house.
Voles are similar to moles....creepy little faces.....we have tons of them in the fields, but have only seen mice in our house. It doesn't take but the tiniest of openings for a critter that small to get in.....I'd be looking around tho, to see what you might have in your house that is attracting them. They like eating roots and grass....where mice will eat anything, so go anywhere to get it....
Well, I don't know a lot about voles. However, I have heard that they can climb, and squeeze into small gaps in your house that are less than an inch wide. So, basically they get into your house from poor foundation, poor architecture, damaged house, etc... I hope this answers your question. :)
With most rodents if they can their head through the whole, they can get the rest of their body though. And their heads are pretty small :P.
Whats a vole?