As somebody mentioned earlier, the best thing to do is to get 1 or 2 of those plastic bait stations you see along industrial buildings so you can safely put some poison outside. I think I've seen them before at some Home Hardware, Canadian Tire, etc., or you can sometimes buy them from a pest control office.
The blocks of poison are secured inside so they can't be dragged out, and cats, etc. can't get to the poison inside. The poisons you get in the store (one with bromidialone is best) don't build up a lot in fatty tissue, so a cat, etc. would have to eat a lot of voles before it's dangerous.
In 6 years in pest control this is the easiest, most effective and safest way I know of.
There's other stuff you can try, like deterrents like ammonia or cayenne pepper, but I don't find they work that well.
This will work for voles, but not for moles.
Snap traps really shouldn't be used outside, they can really hurt other wildlife like birds, the cat, etc.
good luck