White vinegar is good for cleaning some things, it's a mild acid. But what area of the country do you live in? How cold does it get? How big is the shed?

I wouldn't keep a cocker spaniel outside, they are such people dogs, and have been companion animals for so many generations ... if they cannot be with people at your house, I would contact a rescue and re-home them. As painful as it may be for you, please consider their quality of life.