Vaporeon isn't a bad mon, it's just a bad case of being thoroughly outclassed. It's not even as bad in UU as many people say it is and you can use it as a bulky water and it will do fine. And because Vaporeon doesn't have any noticeable flaws to it it kinda deceives players into believing Vaporeon isn't dragging your team down as you can switch it with a more fitting mon. Most people on the ladder assume the tiers more or less reflect pokemon's viability and so just assume all the bulky waters in UU must be of similar caliber. I'm more surprised Alomomola isn't UU already than anything.