If he's hissing at the litterbox...
Is it in a proper location? Every litterbox should be set up so there are multiple ways of escape. They should be open enough that a cat can tell when the troublemaker is approaching. If you have a litterbox in a room with only one door, putting a cat tree is a good way for cats to escape.
You NEED one litterbox per cat at LEAST. If one cat is dominant it could block the other cat from going to the bathroom or attacking the other cat whenever it's using the box.
Cats use pee and feces to mark territories so if you have two using the same litterbox, Newton may be feeling threatened by another cats scent. Have you noticed Vinnie lounging by any doorways, facing inside the room? If he does, then is Newton as far away from the doorway as possible? If so, Vinnie is being a controlling dominant and preventing Newton from escaping the environment. Which is a problem if the litterbox is in that room. (Or vice-versa)
Do they both have access to windows? Patio? Fenced yard? If one cat saw/smelled an intruder they could redirect their aggression towards the other household cat. If it happened at the litterbox, Newton could be scared of it. Even if he was the attacking cat he could feel it's a bad place.
Another possibility is, since Vinnie's six months, Vinnie is growing into a contender for the dominant cat in the house. Newton could feel threatened by this and may be attacking him due to his uncertainty. Having another cats scent in the litterbox could be a sign, to Newton, that Vinnie is taking over his territory. Putting up more cat trees, shelves, scratching posts, and litterboxes may relieve the tension a bit. If possible, and if you feel Vinnie is not safe, separate the two cats and reintroduce Vinnie but this time with two litterboxes and a bunch of places for the cats to escape to if they feel threatened.
Um... I think that's about the only advice I can give you...