Why did my cat who is otherwise healthy just vomit?

He is a 6 1/2 year old brown tabby, neutered. We rescued him and now adopted him. We feed him Blue Buffalo Finicky Feast, along with Whiskas Temptations for treats. We've been feeding him these since last November, and he's never vomited. Ever. He is not an outside cat at all and he's otherwise healthy. When we first rescued him, he was a little imiciated, with worn down mangled claws. Anyways theres a little info on him. Any help is appreciated! Thanks!