The Vayne buffs are a false alarm, there are a lot of angry threads about the new Star Guardian Skins, but none of that has to do with actual game.
New Champions:
Kayn: Edgelord guy with a Scythe. Every game, he eventually gets access to his choice of one of two forms. The red form gets spellvamp, a knockup, and %HP damage, the blue form gets bonus magic damage, and some minor buffs to help him be a better assassin.
Urgot: If he hits you with his abilities, he can activate a minigun that will automatically target you. He has six shotgun knees, which do AoE damage and hurt, but have a large individual cooldown. His ult slows, but if you get below 25% HP, he can reactivated it to supress and execute you.