I'm not sure I see it that way. Vengeance may mean many things, some less literal than others - I mean, what's a better "in your face" revenge on an enemy that wants to destroy the world you worked to create than to ensure its survival or guarantee its chance to thrive, for example?
Dunno.... I just don't buy Mythal as driven only by straightforward vengeance, just like I don't buy that it will turn out that Solas plan is just plain "stupid". There's a lot we yet don't know, even if some clues in the story suggest that things will either go in one direction (some fundamental change to the world prophesied and hinted at since DAO), or that there are more forces at play here than a few would-be gods with a chip on their shoulder on the loose.
I get that a lot of people adore Flemeth/Mythal. I don't hate her or anything. I just don't see her as ultimately being up to any good. She's pushed to help the heroes, sure, but she's always implied that this was for her own ends. We never really knew what those ends were. We still don't, but she was pretty heavy-handed about wanting vengeance in DAI. The reason I think that she might want the Veil taken down to free the Evanuris is because what Solas did wasn't vengeance enough to satisfy her. He consigned them to eternal torment, and that wasn't enough. She surely knows what he did, surely she understands that is a terrible vengeance to have wrought--and yet, she's still trying to exact vengeance. I fully believe that has been her goal all along.
There's also the matter of asking her why she didn't help the elves--and she responds that you don't know what you're asking. Well, okay then, explain it. Or is it because she's in with Solas and wants to remove the Veil? Because I think that's part of it--that's why she gave him her power. She wanted the old world restored. It's not to say she doesn't care about the present world, but that's probably her way of helping the elves, just as it is Solas's way to help the elves. The problem is it doesn't really help modern elves, does it?
Someone even mentioned maybe using all the modern elves as fuel for a massive blood magic ritual. That is not something I can see Solas doing (because Fade), but it's something I could see Flemeth/Mythal having knowledge of. Whether she'd do it or not, I don't know. I repeat that something, even if misunderstood, within Flemeth's grimoire was shocking and horrible to Morrigan in what it involved, even beyond Flemeth taking her daughters' bodies. Morrigan was not a soft personality back then, nor is she now. Whatever was in there, it was terrible. She might have misunderstood the purpose, but does that matter if the means to the end are awful? Much like Solas's plans, the end result may actually be desireable, but is it worthwhile if you have to become a monster to achieve those results?
I do wonder if part of the reason the evanuris all went bad is that old saying, "Absolute power corrupts absolutely." In the end that includes Solas and probably Mythal too.