*facepalms* oops sorry, forgot this was assault vest only. Now that you mention it, idk if reuniclus would be viable for it anyway since a lot of its moves are support based (simple beam, trick, recover, etc.)... something like delphox with fire blast/flamethrower, shadow ball, grass knot/solarbeam, and psychic/psyshock is awesome for an assault vest coming off a base 100 SpDef.
That's laughable, because Delphox has little bulk outside of AV, and slapping AV on it won't help it take most hits any harder than it normally would. That, and the fact that it's SR weak, has a terrible defensive typing in fire/psychic, and generally loses in any contest of bulk make it far less useful as an AV utilizer.
Reuniclus, on the other hand, get reliable recovery in Regenerator, has an acceptable defensive typing, and has the balls to stay in on Gengar (even when mega'd!) if need be.
Bold Reuni vs Timid M-Ggar:
252 SpA Mega Gengar Shadow Ball vs. 252 HP / 4 SpD Assault Vest Reuniclus: 246-290 (58 - 68.3%)
Calm Reuni w/ 218 SpD invest:
252 SpA Mega Gengar Shadow Ball vs. 252 HP / 218+ SpD Assault Vest Reuniclus: 176-210 (41.5 - 49.5%)
and OHKO's back with one SR or spikes switch/ straight off with luck:
40 SpA Reuniclus Psychic vs. 0 HP / 0 SpD Mega Gengar: 254-302 (97.3 - 115.7%)
Delphox can be 2HKO'd with MAXIMUM INVESTMENT and pos nature, and but 2HKO's back. If you get SR'd and don't remove them AV Delphox is useless, whereas Reuni can still switch in and out. This meta is based on pivots and powerful hits, something Delphox can't take repeatedly or dish out against many threats.
You might get a surprise kill if you bring it after something else dies (even then, Volcarona will probably win since it generally runs Fire Blast > Fiery Dance and will likely be at +1 by the time you switch in, and Mega Gengar rarely runs Dazzling Gleam, while Sludge Wave will do massive damage), but your Chesnaught is going to be crippled to the point where it'll be useless in checking pretty much anything else.
Is this assuming that Ches has a niche in this meta AT ALL considering we see so many fire falcons hovering about, waiting to SR up on your 4x weak self? Sorry, but I don't see him being a threat running even the physically defensive sets.
EDIT:
The only users of AV should be bulky attackers plagued by an inability to take special hits, or attackers that would benefit as much as or more from a boosted SpD than extra attack or the like. Regenerators get a niche with AV as a group, and so do Pokémon that have a powerful presence but are plagued by a lack of defenses or have a bad typing. This would include Metagross (maybe?) Scizor, Celebi, and a few more I can't think of.
Adding to that, the Pokémon must also be able to passively boost or be non-reliant on boosting. This item is tricky because most Pokes that can use it are supporters, can't attack effectively without a boost, or suffer from a lack of a different item. This is certainly a niche item that can't just be slapped on a starter with a nod and a wink.