I think you just have to decide how bad you want the horse. Some owners around where I live never vaccinate their horses, never float their teeth, etc. Some people have so many they just do favorites.
I have bought many horses, and can't even say one time that the horse seller gave me paperwork of shots/floating/vet care. I also have to say I asked and been told yes or no, but who knows if they ever did.
If she told you the horse was last vaccinated in July 10, then the horse needs to be vaccinated again anyway. Most folks, if they are selling a horse and it needs vaccinations, then they want to sell the horse before they have to spend more money.
You can't force a seller to have the horse up to date on shots/worming/shoes. You either buy the horse or don't. Just plan on spending the extra money to give shots, etc again. It won't hurt the horse since it sounds like it has been almost a year anyway.