It epends on the breeder. As stated above, a reputable breeder will take a dog back at any time, for any reason. Depending on the circumstances, you may or may not get any $$ back.
As for vet bills...generally only those for certain problems would be covered and only for a certain amount of time and then, maybe only a portion of the bill.
A reputable breeder will do everything within their power to produce healthy pups. And will be reasonable in giving a health guarantee. But, as buyers, we have to remember that we aren't talking about a computer or a car...we are talking about a living creature. Things happen even in the best of breeding programs that just can't be forseen or be blamed on the breeder. We breeders can't be responsible for everything, but the good ones will be there to help should a problem arise.
If you don't get a contract or a health guarantee of some sort...I'd go else where because then you have absolutely no hope of getting any help or support from the breeder. And you can be pretty sure that they aren't worth a hill of beans and neither are their breeding practices.
Just my .02 worth....