Your raise good points. Out of curiosity, do you know the sets for each of those Vivillons?
My Shield Dust comment was harsh as I forgot of Vivillon's other abilities; on reflection I would agree that Compound Eyes and Friend Guard are generally better abilities.
There is 4MSS though, since it wants Tailwind, Sleep Powder, Rage Powder, Protect to block Fake Out and Hurricane. Powder is very difficult to find room for. It's still difficult to warrant over Amoonguss who is more consistent with Rage Powder + Sleep + Protect + STAB, and Smeargle who has more movepool options. Both also have a 100% accuracy move without an ability boosting it.
I am still against it being ranked, but I would not veto it being in D.
Michael Groshan's Vivillon was Rage Powder / Powder / Endeavor / Protect with 0 IVs in HP. From the 3 matches (6 games in total) that were streamed featuring him, he seemed to bring it about every time and use it as a "suicide lead" to ensure Trick Room goes up. With Rage Powder, Vivillon is usually guaranteed to either go down to sash or get KOd; if it goes to sash, Endeavor makes it threatening, and if it gets KOd, it's a free switch for the TR sweeper of his choice.
I think this last part shows this Vivillon's niche. Amoonguss is a much better Rage Powder user, but it tanks hits, which isn't something you want from a suicide lead. He didn't want to stick around and put things to sleep like you would do with Amoonguss, so in that sense, I think the only other Pokémon to be able to fill that niche well would be Smeargle, which can literally run the same set. But I assume Vivillon was chosen because of Friend Guard; P2 was a common TR setter next to it, and a Friend Guarded P2 has ridiculous bulk if Vivillon decides to stick around instead of Rage Powdering. So Vivillon can make sense on some teams over Amoonguss and Smeargle, though in most cases the latter 2 are probably better.
I just checked the VOD for the other Vivillon user and it happened to be Gary Qian (I guess he's the kind of player you'd totally expect would be running Vivillon lol), he was Compound Eyes with Sleep Powder / Rage Powder / Hurricane / ??? (I assume this was Protect, but he didn't Protect in front of an incoming Fake Out, so it may have been Tailwind?). This is more like a standard set one would expect.
As a Tailwind setter, Whimsicott (and even Illumise) is surely more reliable, and can also use a Sash + Endeavor option (as well a myriad more tools), but doesn't have a good sleep-inducing move, and no Rage Powder, so I can see why Vivillon could make sense, it depends what support tools one precisely needs for their team.
It definitely suffers from 4 MSS as you mention, and will always miss out on a potentially useful move, but I'd say Whimsicott has the same kind of 4 MSS, wanting Encore, Tailwind, STAB, Protect, Fake Tears, sometimes Endeavor, sometimes Beat Up... that's part of what makes it scary to face, though, because you may have to try and play around a move it doesn't even have (though that's less of an issue in Bo3).
Point in case, I think Vivillon has a small niche of its own because of its abilities and the unique combination of support moves it has. In most cases, something else will be better (Amoonguss most of the time). As pointed out earlier, a large portion of the metagame it well-equipped to deal with it, between Rock Slide and Misty/Electric terrain. I didn't have much of an opinion about ranking it yesterday, but from looking more into it now, I think it fits the description of D-rank pretty well, so I could see it there.