Victini is not even OU,it's UU,smogon's reason is absolutely correct..I don't see what's so wrong in it.It doesn't matter if a Pokemon is a event legendary or it has wide pool,it's usage in metagame and how good it is is what matters.Victini is not that good at all...even in UU it's walled by stuff like Slowbro and Milotic which are not 2hkoed even by Sun boosted V-create.
252 Atk Choice Band Victini Fusion Bolt vs 252 HP/252 Def Slowbro: 46.7% - 55.33%
2-3 hits to KO (with Leftovers)
Fusion bolt will make Vicutini a bit dangerous,but then again those are calcs of a banded vicutini which means you can easily switch into a ground type,while slowbro heals off with regenerator.UU is infested with bulky waters,so you won't be having much problem handling Also bulky intimidate Arcanine also checks it well.
As for the sets,I liked the 2nd post,it covers most of the vicutini's movesets.While Vicutini's stats tempts you to use a defensive set,note that fire is a good offensive type and not defensive,so you are better off with offensive vicu,defensive set isn't worth it...it's movepool just supports this fact
Well, V-Create 2HKOs Milotic.
Adamant 252 Atk Choice Band Victini V-create vs Bold 252 HP/ 252 Def Milotic: 63.96% - 75.38%
That's a 2HKO even with Leftovers. And an OHKO with 3 Layers of Spikes + Stealth Rock.
Slowbro, on the other hand, is 2-3HKOed because of Leftovers.
Adamant 252 Atk Choice Band Victini V-create vs 252 HP/252 Def Slowbro: 51.78% - 60.91%
Throw in Stealth Rock and Slowbro goes down.
Finally, Adamant 252 Atk Choice Band Victini V-create (-1 Attack) vs Impish 252 HP / 252 Def Arcanine: 43.49% - 51.3%
Stealth Rock go boom!
Also, by having that EV spread, Arcanine's offensive capabilities are zilch. I know that you're not suggesting Impish 252 HP / 252 Def, but any less and Arcanine's going to be 2HKOed before it does anything.
Fusion Bolt: 100 Base Power x Super Effective = 200 Base Power
V-Create: 180 Base Power x STAB x Sun Boost x Not Very Effective = 202 Base Power (Rounded Down)