How do you stop cat peeing in vents?

First, take the cat to the vet. If there are no urinary tract or otherwise problems, place litter boxes in places where he/she is currently urinating.

If you can't do that (like if your cat is doing his/her business where you have to walk), make the place where he's going as unattractive as possible, and the spot where he's supposed to go as attractive as possible. Turn the place where he's going into a feeding station; he won't do his business near his food. Place the litterbox in a quiet, easily accessible place (young kittens and senior cats don't like going up and down stairs a lot). Remove every bit of evidence of his urine from the place he's been using (don't use cleaning products containing ammonia, they'll smell like pee to the cat and he/she will attempt to cover it by peeing again), also, you can temporarily put aluminum foil in the place where the cat's not supposed to go (they don't like the sound or feel of aluminum foil, so they'll avoid the area. Also, spraying lemon scent will repel a cat.