Everything @jirachi123 has said and - Parallel City also reduces Volcanion's damage. So if they miss a steam up, or are forced to use a couple of extra steam ups the math will throw the game to you long term energy wise. Take a Zoroark for instance: It takes 3 steam ups + a fury belt, or 4 steam ups in order to take a ko when parallel city is up. And Vespiquen should be running buddy-buddy rescue at this point - so if they target down a vaporeon, not only does that give you an extra dce on the board it also gives you extra options later down the road giving you an even bigger advantage. It is favorable, weakness is a bit of trap/mirage that can look daunting.
For reference - The other Vespiquen decks that made t8 at Athens never lost to a Volcanion either - and they weren't even running Vaporeon, the worst that one did was a tie. Weakness doesn't necessarily mean much outside of potentially psyching someone out