By scaling, she was likely the weakest triad member.
Noelle being her initial opponent upon introduction was pretty telling, given that Noelle has pretty consistently been portrayed as the weakest of the main three characters (not by that much, but enough to set a noteable precedent). Consider the Royal Knights exam, where she was second fiddle on her team to Yuno, or the Elves arc, where she needed help to win against her apostle, whereas both Asta and Yuno would go on to face against the main boss.
Vanica was definitely portrayed as superior to max power Noelle, given how much Lolopechka helped her in that fight, but Dante and Zenon were definitely portrayed as superior to their opponents as well, who themselves had better scaling.
I mean, she also had the feat of killing Noelle's mother, who had ludicrously high scaling for the beginning of the series, but Black Clover power-creep being what it is, I don't think that's enough to put her above the other Triad members.
In terms of devils, as stated earlier in the thread, Magicula was clearly the weakest of the three that the triad possessed, however, one thing that is in Vanica's favor was that Magicula was clearly much more involved in Vanica's affairs then the others were with theirs. Moreover, given how close Magicula was to finishing "Malevolent Femcantation", in practical terms, Vanica was the Dark triad time bomb with the shortest fuse amongst the three, and the largest inevitable explosion. But that stuff isn't actually "Vanica's" power given how she'd have nothing to do with it by that point, so she still probably counts as the weakest.
Thus, the final thing to look at is Hax, and how OP Vanica's abilities are compared to the others. Blood magic, as we've known since the witch's forest, has some unbelievably powerful healing spells, as well as a remarkable potential for body puppetry hax. Curse Warding magic, as well, is clearly one of the most versatile attributes in the power system. Unfortunately, these two attributes possess a rather remarkable degree of overlap, so in terms of character spec, Vanica is kind of inferior when theoretically she should've had an edge.
Curse Warding magic and blood magic are also best utilized in scenarios outside of direct combat, which also synergies poorly with Vanica unreasonably high bloodlust. This does mean that her power potential is likely much higher than what she's shown, should she commit to more subtle and insidious hit-and-run tactics, but given that her character would likely never do or utilize something like that, even under the most generously dire threat of death that I could reasonably imagine for her, I cant really count that in her favor.
Though I still think that she's probably a bit stronger than the likes of even Yami overall, and her presence is ultimately the most dangerous of the three, I definitely think I would put her down as the weakest of the Dark Triad.