[{quoted}](name=Arya Riven,realm=NA,application-id=3ErqAdtq,discussion-id=U70ob3nn,comment-id=,timestamp=2017-01-26T23:14:37.981+0000)
Both are "high skillcap",
HAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHA!
But seriously... Do you know how if you mix two kinds of awful, corrosive, acidic chemicals you'll likely either cause a horrible explosion or a cloud of toxic death fumes? That's pretty much how this works too.
See, Riven is basically Chlorine. She's pretty commonly used by people, but she's also really toxic and being around her is bad for your health unless she's very properly treated.
Vayne however, is Ammonia. She's gross and prone to burning people who mishandle her, but can be useful at cleaning up things when other less extreme options just won't work.
But mix the two and you're dead. Their toxicity just does mix. The fumes that result will kill you and everyone on your team.
Now... some toxic materials CAN be mixed though. Yasuo and Riven for instance, both mix fairly well. I mean, they're invariably toxic as all hell, but the players who use one often use the other. Some believe this is because Yasuo is actually a base, not an acid. This would explain why he mixes so well with so many other toxic champs. This is entirely speculative though, as the resulting player is almost always still incredibly acidic and toxic, where one would otherwise expect a neutral compound if he was somehow neutralizing the inherant acidity of a pubstomping toxic shitter champ like Riven.
Ultimately, Riot just hasn't put enough effort into it's chemistry department for us to know the reasons for sure. Just remember to wear safety goggles while experimenting with toxic champions, if you must experiment with them at all.