Here was once a quote answer, where I misunderstood something that Azure Demon posted. Sorry about that one.
Aside from that, I will post my thoughts here about Victini and it being bannable vs not being bannable too. At the current moment of time, after reading some discussion and starting off with this opinion; I am leaning against banning Victini.
The reason why I do not think Victini should be banned at the moment is the same reason why Kyurem-B and Keldeo are still OU. Before I start comparing these too much, I think I should state my opinion on when / how something should be considered bad for the metagame. I also think I read this in some Ubers / OU post, so sorry if this isn't completely my analysis of offensive broken threats.
There are three types of offensive threats in my eyes.
· First of all, there are those threats that can 2HKO almost everything (Examples from all tiers are: Sawk in NU, Aggron and Escavalier in RU, Zapdos and Victini in UU, Keldeo and Kyu-B in OU, and Kyogre in Ubers). These threats are often seen using Choice Band, Choice Specs, or Life Orb. They are also known as wallbreakers. The way I see you can measure justification of banning these are a combination of answering these two questions; How easily is it revenge killed? How many Pokemon can counter it consistently? I'll try answering these about Victini in a bit. Ex. of a broken wallbreaker that got banned: Torn-T got banned from OU.
· Second of all, there are the threats that can, if it finds free turns, potentially sweep entire teams (Examples from all tiers are: Gorebyss and Scolipede in NU, Kabutops in RU, Azelf in UU, Dragonite in OU, and Arceus-Normal in Ubers). These threats use boosting moves, sometimes with Life Orb for even more power. The way I see justification for banning these are a combination of: How easy does it set up? and Is it fast enough to really sweep an entire unprepared / weakened team? Example of a broken sweeper: SubSD Garchomp was banned when Sand Veil was legal.
· Third is revenge killers. I don't think this role by itself is justification enough for banning, but it both helps deal with above mentioned threats, and can also (most of the time) perform said roles as well as revenge killing. If a Pokemon can perform all three of these roles, and perform them consistently, I think it gets even closer to being stapled as "broken". Oh look, Genesect got banned for pulling off both role 2 and 3 extremely well.
And then a final question that goes for all of these: how weak is the Pokemon to common strategies of the metagame? Entry Hazards everywhere, Weather and Pursuit-trapping in OU, Bulky Offense in RU (kinda idk).
When it comes to Victini, it is definitely fitting for the first category. Both it's Choice Band, Mixed, and Special sets are extremely hard to switch into (as stated by above calcs btw). So let's ask ourselves the questions.
· How easily is Victini revenge killed? Well, to be honest, Victini really just is not hard to revenge kill. Common Choice Scarf users such as Mienshao and Flygon can easily KO after Stealth Rock + Defense drops from V-Create. Pursuit from Honchkrow and Krookodile completely annihalates Victini (It also cannot play around Sucker punch from krow and bisharp).
· How many Pokemon can counter Victini consistently? Rhyperior and Swampert can tank V-Creates for days and easily KO / set up Stealth Rock back. Even though they are unable to take Grass Knots, Victini has to either run Life Orb (which wears it down with hazards) or E-belt (which doesn't boost main STAB). Besides, it is predictable if you see the Life Orb, and it should generally be scouted for by double switching when taking the place of Victini's opposing player. Snorlax also is able to wall most sets, and you will need some boos prediction to win against it, especially since it has Pursuit most of the time. Overall, you also really have to predict well when using Victini, or you might end up in an extremely awkward spot (think offensive Zapdos switching into a predicted Grass Knot after you V-Created a rhyperior; ur fuked).
· How weak is Victini to common strategies of the metagame? Entry Hazards are everywhere, and Victini can take up to 50 percent from it. This means that, with correct prediction and setting up rocks / spikes / both will keep Victini somewhat in check by itself, especially if it is wearing itself down by Life Orb. Pursuit-trapping is also semi-prevelant, and with Snorlax (possibly the most popular one) can resist V-Create, and generally be a bulky bitch, it is also easily trapped
tl;dr, I don't think Victini should be banned, because I think it is worn down / revenge killed easily.