This is now my exact same attitude towards the non-main-manga materials. Though, as time goes, I think more and more that Vigs is the biggest offender. Reason being, compared to other non-canon materials, Vigs tries the hardest to insert itself and influence the main story, while having huge glaring problems, such as:
- Nonsensical "cameos"
- (Aizawa, Midnight, and Mic appears in every turn that the Vigs trio is involved because "why the heck not?"). It feels more and more that the main-story characters are just being dragged into Vigs to try to appeal to the main manga audience, ("hey check this out, we have the story for Stain! Iida Tensei! Fat! and Aizawa!"), not to serve these characters a decent and satisfactory story.
- Characters acting OOC
- For example: Aizawa is aloof, not snarky like what Vigs shows; Endy is hotheaded, but he's not an idiot. There's no way he would almost accidentally burned a villain that looked like a living bomb; AM is clumsy, but there's no way he'd be so clumsy that he would reveal his identity to a person he just met--which, according to Vigs, this is how Tsukauchi, AM's police friend, found out AM's real identity.
- Powerscaling that does not fit the main story
- Some random American hero can fly and have super strength but for some reason he had zero impact to the society. Some random hero can accelerate himself so fast that he could become imperceptible, clearly beating out all the speedster heroes in the main story. Some villains in Vigs just outright have regeneration when in the main story, AFO had been hunting for this quirk for a while.
- Outright weird timeline.
- Vigs happen in the past, but not too far in the past, a lot of people estimate that Vigs happened while the main cast is still in middle school. But for some reason, Midnight, Aizawa, and Mic haven't become teachers in UA at this point. Meanwhile, in the main story, Aizawa had made a reputation for being a very strict teacher who will expel students left and right, going as far as expelling an entire class. Is this reputation supposed to be built in less than 5 years?
- AM and Tsukauchi is very familiar with each other, in the main manga, Tsukauchi went as far as saying that "We will capture AFO for sure this time", indicating that they must have known each other before AM fought AFO for the first time. But for some reason, Vigs says that Tsukauchi and AM met only after AM fought AFO and become skelly-might. Again, inconsistent with the main story.
Comparing Vigs to other non-main-manga materials: SMASH!! is clearly a gag, so you're not supposed to take it seriously. TUM, so far, looks like a lighthearted story with zero plot relevance, kinda like the School Briefs/LNs but in manga form. The movies fill in short stretches of time that the manga doesn't cover, basically like every big shonen movies does.
Hori hasn't
explicitly shown any plot points or characters from Vigs. The quirk-booster wasn't named in the main story, the Naruhata Vigilantes was never mentioned, let alone name-dropped
or appear in the flesh. The additional materials are not to be considered canon, especially in this kind of work (battle shonen) where additional story is made to cater to the audience, not to serve the story. Canonicity is like a handshake, the main story has to acknowledge the spinoff, not just the spinoff just taking plot points that the main story generated. So far, what Vigs has done is taking story elements that the main story has thrown, like Fat's "mission in the past", and now Aizawa's "past". Until Hori dispels this "plausible deniability" and "accepted the handshake", Vigs is
not canon
Since when does the main story have to acknowledge a spin-off of it to be legit? This isn't the MCU. There's no ultimate team-up between the Vigilantes and Deku coming. This is simply a story taking place in the same world years before the main series starts.
Continuity doesn't necessarily work like a handshake. Stories can take place in the same world and not reference each other. This isn't fan fiction. It's a legitimate story happening in the MHA universe under the consent of Horikoshi and published under the same parent company as the main series.
I don't know what else to say. Yeah, Vigilantes is expanding on throwaway lines said by certain characters and giving others a more fleshed-out backstory. Lines and characters from the main series. What more do you want? The Crawler showing up randomly in MHA to go bowling with Aizawa? As I said before, this is a supplemental series. The main series doesn't have to mention Koichi or the other main characters, that's the point. We get to learn more about the world and certain characters from the main series that doesn't directly impact the main series story. It's additional information, it isn't required reading.
Horikoshi and the official Viz translator for MHA and MHV treat them like they're happening in the same universe. If that's not good enough for you we'll just have to agree to disagree.
EDIT; Guess I'll address more of your points in detail
Nonsensical "cameos"
You're right about characters from the main series coming in to appeal to the main series readers. That's because people who read the main series are more likely to pick up Vigilantes. Nothing wrong with that. You could say it connects the two stories more. And how is learning more about Iida's brother, Stain, Aizawa, etc. hurting these characters? There's no room to put these stories in the main series that's why they can be told in Vigilantes. They're getting more focus. That's a good thing.
Characters acting OOC
Aizawa taking naps in his sleeping bag at U.A. is him acting aloof.
Endeavor made a mistake, it happens. Like him forcing his kid to be a hero or turning his back on his other kids or making his wife so miserable she hurt her son because she thought he was becoming more like his father.
All Might didn't notice Deku holding on to him in the first chapter and ending up revealing his identity to him. I'm not seeing any OOC here.
Powerscaling that does not fit the main story
Captain Celebrity was a total jerk until recently. He wasn't in it to help society as much as helping himself be famous.
O'Clock was fast, yes, but we don't know how good a hero he was. We don't know his ranking or how long he's been inactive since Vigilante's first chapter. It seems like you're implying these guys should've stopped crime period. If that's the case, why is there crime in the DC and Marvel universes? You seem to be holding them to an extremely high standard. All Might wanted to be a symbol of peace. These two probably never even thought about it or couldn't because they have other priorities like their families.
Regeneration? Are you talking about No. 6? If so, then AFO already has access to that power because it's highly likely he's part of AFO's organization. The Villain Factory is what the police is calling it.
Outright weird timeline.
Midnight is a teacher and Present Mic will start next semester. Yes, Aizawa isn't a teacher yet, but there's been no official date when Vigilantes takes place and we don't know the circumstances under Aizawa expelling those kids. It's not weird, it's unexplained.
Sounds like you make these arguments without reading Vigilantes. If I'm wrong, please let me know.
And since you brought up the movie. Horikoshi gave the next movie's main villain a brief cameo in MHA. Under your handshake argument, that movie's canon. Explain to me how that's canon and not Vigilantes even though multiple characters from the main series are in it and nothing so far has contradicted the main series?