I hadn't thought about it, but you're right. Breaking through the Vignette is not likely where the story ends, I can't imagine the witch council or whatever will be happy just watching Marianne and a flood of her students just run wild. I think in addition to having a clear goalpost for the story, Vignette Witch has a nice mix of both mystery and suspense.
Suspense being, yeah, what happens if/when they break through the Vignette. if Konpachi's feminization magic is totally a subversion of the rules of this world, where did it come from? Will it remain a talent exclusive to them, or will the others gain access to it? Or... God help us, what if Marianne ultimately does? Does she really only want disciples with the intent of breaking through the Vignette or does she have a larger plan? I can't imagine she'll just want to go back on the lam.
On that note, Mystery. We have no idea Marianne's crimes might actually be. I find it unlikely she's being truthful about it just being a misunderstanding, but it's also not like the ones jailing her are being remotely reasonable either -- those were some seriously total war tactics. It remains to be seen if they resorted to all the collateral damage due to a moral failing or if it was a Godzilla Threshold level decision they were willing to make in order to contain Marianne.
So yeah, unlike TS school which kinda just stumbled through introducing its premise, Vignette Witch very much laid some very solid foundations. I think we're going to see a similar storyline play out too, with different boys one by one accepting their fate as witches. Like a more downbeat, intense version of TS school's conflicts.
(but almost certainly occasionally uplifting because that's how Kaneko Naoya tends to write TS characters; expect at least some of them to be eggs, mark my words. That line about beards vs ribbons that exist in a guy's heart is going to come into play in some way beyond just poetically alluding to Konpachi's transformation.)