edit @ spino: You do want to keep Rotom-W thanks to handy resistances, a good defensive typing, good STABs, etc. I'd rather have Shadow Ball/HP Fighting > HP Fire because of Drizzle, though. As far as Gorebyss, it's not necessary but I'd most likely put it over Ludicolo who basically just sits there and tries to get something done but doesn't do much. The set looks nice (Leech Seed + Rain Dish recovering full health subs!) but it's really just not effective and things set up on it and kill it too easily. Also, Acid Armor is useful as it gives Vaporeon pretty solid defenses and makes it easier to take hits/recover easier. It's standard on the Hydration set, anyway. Wish is incompatible with Hydration.
If he does end up putting in Gorebyss, he's got his Thundurus check.