It is completely unacceptable that the adopters would return the kitty to you without providing a cogent explanation for the horrific injury. Don't let them off the hook that easily!
I would take your kitty to the vet ASAP, if you haven't already. The vomiting and possible nausea can be symptoms of so many things--some more serious than others. And chronic nausea is a horrible condition for an animal to suffer. Also, cats are experts at concealing pain and injury. You won't...