Vittoria gets the play rolling: she's at the center of the action, even if she's not quite as constant a presence as her brother, Flamineo. Basically, she's cheating on her husband Camillo, with the Duke of Brachiano. Camillo's connected to important people (like the Cardinal Monticelso, his uncle) but, like Vittoria, he's poor. And he's not satisfying his wife's… needs. (If you catch our drift). So Vittoria falls in love with Brachiano… who's a Duke, a real live Duke, no less. Brachiano loves her too—and dislikes his own wife Isabella. So, Brachiano and Flamineo (Vittoria's brother), and probably Vittoria herself, hatch a plan to murder Vittoria's husband and Brachiano's wife. It works, but Vittoria gets arrested.
In court, Vittoria aggressively defends herself. Camillo's uncle, Monticelso, attacks her during a long speech ranting against "whores," and makes other nasty comments. She defends herself with intensity:
Mont. I shall be plainer with you, and paint out
Your follies in more natural red and white
Than that upon your cheek.
Vit. Oh, you mistake!
You raise a blood as noble in this cheek
As ever was your mother's. (3.2)
But despite giving a really good performance as a brave young woman unjustly persecuted by the Cardinal and his allies, Vittoria actually might be totally guilty. We know she's lying when she bold-facedly claims before the court that she wasn't about to be seduced by Brachiano, and the dream she tells him in Act I Scene 2, helps inspire Brachiano to arrange the murders.
Webster's messing with us—he wants to provoke us to sympathize with Vittoria as a victim… and then suddenly tear the rug out from underneath us, when we remember her guilt. She's a weird counterpoint to Cordelia from King Lear—whereas Cordelia is purely virtuous, and really is treated extremely badly by her father, the king, only to (spoiler alert) die as a sacrifice to the chaos created by her father's pride, Vittoria is a counterfeit Cordelia. She might quack like a duck—but isn't that just a hunter's duck-call whistle at her lips?
Yet Vittoria's qualities of courage are real—it's just that's she's a pretty lousy human being. Webster's showing us that something admirable can be part of the makeup of a basically bad person. You can probably think of a professional athlete who's done something utterly horrible—but the horror is made remarkable by the real qualities of determination and hard work demonstrated by that athlete. Those qualities don't make the athlete's actions better, obviously—they make those actions seem all the more messed up. We feel odd thinking about the contrast. Vittoria—this woman full of energy and zeal, yet seemingly murderous—is a good example of the same kind of thing.
In the end, after being imprisoned in a convent and then released, Vittoria finally gets murdered. Her lover, the Duke, has just got his, and now it's Vittoria's turn to get her (probably) just desserts from Count Lodovico. As she dies, Vittoria cries out:
My soul, like to a ship in a black storm / Is driven I know not whither. (5.6)
Since a ship in a "black storm" doesn't exactly sound heaven-bound, we can assume Webster is implying that she's headed for hell—thus making her guilt fairly clear. Her final words provide a good epitaph for herself, sealing her as a cautionary example for the ages:
Oh, happy they that never saw the court, / Nor ever knew great men but by report! (5.6)
Yet, in a way, this throws her own guilt off on someone else—on Brachiano—instead of facing up to facts. Brachiano does the same thing when he thinks Vittoria's cheating on him, and blames her beauty for leading him into evil (4.2). Part of the psychology of these characters is that they refuse to admit who they really are, and always cast blame on others. Flamineo is an exception—he recognizes his own evil.