Tiers are based on usage so Pokemon that aren't commonly used or voted broken will fall to lower tiers.
Victini's flaws often out way it's strengths due it's now mediocre defenses due to the infamous power creep with masses of key weaknesses brought by its typings. Also as far as I'm aware Celebi, Mew and Jirachi also happen to be UU this gen so why wouldn't Victini?
Uh, no, pokemon who are voted broken are kicked to Ubers.
Jirachi is OU.
Moving on, I'm surprise no one has mentioned this set, as I found great success with it. Too addicted to V-Create, I suppose.
Victory Star Abuse
Victini w/ Victory Star + Choice Scarf/Specs/Life Orb
Modest/Timid Nature, 252 Sp Atk, 252 Speed, 6 HP
Fire Blast
Psychic
Focus Blast/Final Gambit
Thunder
So. Your main attack here is going to be Fire Blast, a tremendously powerful STAB move which Victory Star grants almost perfect accuracy. Oh, yeah. It has a 30% chance to Burn, the same as Lava Plume. Meaning you have an almost 1 in 3 chance to burn with a strictly offensive move. And with 100/100/100 defenses, any physical attacker who gets burnt will be... well, screwed. Psychic is his alternative STAB, and isn't a bad type to have, considering how good fighting-types are this generation. Focus Blast and Thunder are coverage moves who have their accuracy boosted by Victory Star. As I'm sure you know, Thunder has a 30% chance to Paralyze, like Fire Blast's burn. However, with Thunder's lower accuracy and lack of STAB, you really shouldn't be relying on it. At all. Or even counting on it. Unless you're up against a rain team. Then it's your best friend. Focus Blast is even less reliable than Thunder, but it still has a higher chance of hitting than it does missing. It almost has 80accuracy after factoring in Victory Star (Thunder has about the same accuracy as Fire Blast without Victory Star, if you're curious. Final Gambit is an option on the Choice Scarf set, as even with minimal HP investment, most pokemon will have less than Victini's base 100 HP. Almost all sweepers (Not Honchkrow and Excadrill, though. But you don't want Victini up against those anyways).
Now, who counters Victini? Well, if you know your opponent has Victini, you're going to want to get SR down. No matter his set, it's going to help. Especially if it's a choice set. On the flip side, if you have Victini, you're going to want a spinner. If you have a choice item on Victini, you're going to NEED a spinner. For the Trick Room sets, a steel-type can help you lots. A lower-tier TR-abusing steel? Sounds like a synonym for Escavalier! Victini both takes the fire attacks he has a massive weakness to, and obliterates the steel-types that Escavalier has so much trouble with. In return, Escavalier dominates, well... Everything that doesn't resist bug. Seriously.