There's probably a mention of Assault Vest on here, but I didn't see it. So, Vest Volcanion seems pretty legit, with Water Absorb and a dual Fire + Ice resist giving him decent special switch - ins, and recovery, though it's unreliable. Also, physically defensive could see use to counter Specs Keldeo, with Specs Secret Sword
252 SpA Choice Specs Keldeo Secret Sword vs. 252 HP / 252+ Def Volcanion: 132-156 (38.3 - 45.3%) -- guaranteed 3HKO
and Hydro Pump healing him. And scarf could work, as it would be like a Rotom or Heatran scarf set. However, this thing gets no good special coverage, and is walled by the Lati@s, as well as Chansey. With Specs, he hits like a truck, and most walls fear dat burn. He also has enough bulk to take a Banded Talonflame Brave Bird.
252+ Atk Choice Band Talonflame Brave Bird vs. 0 HP / 4 Def Volcanion: 201-237 (71.5 - 84.3%) -- guaranteed 2HKO
252+ SpA Choice Specs Volcanion Steam Eruption vs. 252 HP / 4 SpD Amoonguss: 177-209 (40.9 - 48.3%) -- guaranteed 3HKO after Black Sludge recovery
That's powerful.
Lati@s would be considered a check cuz I don't think they'd appreciate taking any attack from Volc, much less taking a burn from it. I'm on my phone, so I can't get calcs. Chansey walls every special attacker, so there's no point in mentioning that. Volc doesn't wanna switch in on Talonflame and vice versa. Also, I doubt amoongus wants to switch in on Volc, unless it wants to risk it not being specs or if it gets the burn.
EDIT: I now have access to a calcutator. Hallelujah! Turns out that the Lati twins really couldn't give two shits about Volc, not including Hidden Power coverage. Latias would have a better chance against Volc as it has slightly better bulk, but really, it doesn't matter which one is used.
252+ SpA Volcanion Hidden Power Ice vs. 0 HP / 4 SpD Latias: 110-130 (36.5 - 43.1%) -- guaranteed 3HKO after Stealth Rock
252 SpA Life Orb Latias Draco Meteor vs. 252 HP / 4 SpD Volcanion: 266-316 (77.3 - 91.8%) -- guaranteed 2HKO
With Latios, it's a slightly different story. It has a chance of being 2HKO'd after rocks w/ HP Ice, so it
may have to be a little wary.
252+ SpA Volcanion Hidden Power Ice vs. 4 HP / 0 SpD Latios: 126-150 (41.7 - 49.6%) -- 84.8% chance to 2HKO after Stealth Rock
However, it has no trouble KOing Volc right back. It's a guaranteed OHKO if rocks are on Volc's side of the field.
252 SpA Life Orb Latios Draco Meteor vs. 252 HP / 4 SpD Volcanion: 300-355 (87.2 - 103.1%) -- 18.8% chance to OHKO
Of course, neither one would appreciate a burn in the long run.