I don't think Vanny was ever intended to be anything special by SteelWool

"But the teaser trailer had her talking in it!" It's very common for trailers to have dialogue made specifically for them. Hell, both Ruin and Help Wanted 2 did this in their trailers. They summarize the plot of the game. The original SB teaser did the same. Just less openly. Kid is trapped in the Pizzaplex and is being hunted with Freddy protecting him. I don't think the plot of SB changed as much as a lot of people think it did. It was just being formed slowly and shown to us in an abstract and arguably deceitful way. "But what about all of Vanny's unused animations?" Most of those are alternative walking and running animations. The only stand out is the one that depicts her crawling.

I guess this could have been used, but it was likely replace by the mini music men to add to the claustrophobic nature of the vents. "But what about that Glitchtrap voice at the start of the "Gameplay" trailer?" Again, that sort of thing is very common for trailers. To have audio specifically made to entice the audience. That's what a trailer is for. To make the product it's advertising look as good as possible. Disregarding the hype surrounding Vanny, A new trailer will almost always put a new character front and center. That's just basic marketing. To push the shiny new thing as hard as it can. The trailer audio didn't give us much new info anyway. It was just doubling down on what we already knew. That Glitchtrap was manipulating Vanny. Neither of them were ever meant to have that many voice lines. Hell, there's even an unused voice line of Gregory making fun of her catchphrase because of just how much she says it.

You ever notice how most of the shots that feature Vanny in those two "gameplay" trailers are long, straight and mostly featureless corridors aside from some bright red lights?

That's because they were made just for the trailers. They were made to show Vanny off and nothing more. They were never meant to be in the final game. It seems like for whatever reason, Fnaf wants nothing to do with her. Ruin canonizes an ending where she's less likely to be an antagonistic force in any potential future appearances, Tales from the Pizzaplex doesn't even mention her, and Help Wanted 2 seems to double down on her not being around anymore by showing her literally waving goodbye to the player.

I'm half convinced that the only reason the PQ ending was picked as the canon one over the clearly originally canon Burntrap ending was just so they could write Vanessa/Vanny out of the story.

Not only is the tone from the PQ ending very upbeat compared to Ruin, while the Burntrap ending would feel like a much more natural continuation going into Ruin given that we straight up see part of the Pizzaplex collapsing. Plus, part of why the PQ ending is the happiest is that all the animatronics are freed from Glitchtrap, but what did freeing them actually accomplish? All of them except for Roxy have completely lost themselves and gone insane. All Chica can think about is food even after she gets her voice back, Eclipse doesn't even recognize Cassie and thinks the abandoned Pizzaplex will be "flooded with kids," and Monty has gone feral.

So yeah. The only real hope I have left of Vanny making a come back is her somehow showing up in the fnaf movie or that Vanny graffiti in Ruin meaning that she's gonna come back, but otherwise, I'm just not having a good time man.