shrang
General Kenobi
You're describing a counter. Vaporeon wins against MixMence if you can get him in safely at a decent amount of HP.I don't really consider Vaporeon a check to all of Mence's sets when it gets 2HKO by 0+ outrage, and gets owned by Draco Meteor + Outrage.
Oh and cape, I'm not oblivious to Cresselia having big problems in the OU metagame. However, if Mence is such a big problem, I see no reason why people won't use it. The metagame has shifted away from "A counters B" to "B can be defeated if I do this". This is one of the reasons why Cresselia isn't used any more. Picture something like Suicune, who may lose to MixMence if you predict incorrectly, but otherwise quite a decent check to Mence. Meanwhile, Suicune can do a sweep, tank, phaze and is in general more useful than Cresselia. So why should I use Cresselia, when I can use Suicune and do what Cresselia does ~70% of the time and do other stuff at the same time?? Yes, I might lose to Mence 30% of the time, but that 30% isn't that bad, and losing to Mence doesn't mean you lose the game anyway (So probably 10% of the time, I may lose because I lost to Mence). On the flip side, if I run Cresselia, I can beat Mence >95% of the time, but have reduced efficacy everywhere else. If I really wanted to counter Mence (And Ape, Gyara, Dragonite,etc), I'd use Cresselia. If I want to more than just counter threats and do something with my team, I wouldn't. I'll go back to the Tentacruel example. NP MixApe was a beast back in those days, so much that it brought Tentacruel out of UU. I'm aware that Tenta can support in other ways as well, but the biggest reason was to counter Ape (Forretress can do exactly the same thing Tentacruel does apart from countering Ape). Infernape was one of the biggest threats. Now, if we look at Mence, if he was such a big threat, there is no reason why we can't go into our stash of UUs and BLs and pull out Cresselia or Regirock or whatever. It says more about Mence more than anything.