She first appeared in "Vanessa Meets Doof", a deleted scene which can be seen from the bonus features in the movie's corresponding DVD. Arriving to her dad's building, Vanessa complained to her dad about banishing Tony Marzulo, another boy who showed an interest in her. She warns him that he keeps this up, she will never have a social life. When her dad says "done and done," she notices another person standing next to her, asking her father who it is. Her father responds that that person is just himself from another dimension and tells her not to mind him at all. At that point, Vanessa storms off and says she will be going back to her mother's place, and that she'll be taking her father's blimp keys to get there. (Phineas and Ferb The Movie: Across the 2nd Dimension)
Ever since her father was arrested for his crimes against humanity, Vanessa stayed at her mother's penthouse. She also knows that her mother is just as evil as her father, since she is aware that Charlene had kidnapped several of O.W.C.A.'s agents and turned them into cyborgs, similar to how Heinz defeated Perry the Platypus and turned him into Platyborg. During the battle between The Resistance and Charlene's cyborgs, the Resistance manage to free the agents of their evil programming, and their leader Candace Flynn is about to take both Heinz and Charlene into custody. However, Vanessa interrupts as she arrives to the scene, telling everyone to stop. She then convinces Candace to let her have a moment with her family. However, this was all a trick that Vanessa made up to help both her parents and Pandaborg escape in a hover car driven by Tony, proving that she is somewhat evil. As the Doofenshmirtz family escape, Vanessa introduces Tony to her parents, and Heinz recognizes him as the boy he once banished earlier. (Tales from the Resistance: Back to the 2nd Dimension)
Similar to her original counterpart, Vanessa is sharp and sarcastic, and her behavior is comparable to that of a typical teenager. She cares about getting a social life, having been upset when her father banished Tony Marzulo for showing an interest towards her, warning him that she will never achieve a social life if he keeps this up.
Also, just like her original counterpart, Vanessa did prove that she may be somewhat evil like her parents. To a small yet significant extent, Vanessa tricked Candace into letting her have a moment with her parents, which allowed the entire Doofenshmirtz family and Pandaborg to successfully escape in a hover car driven by Tony himself, much to Candace's anger. This also might indicate that Vanessa still loves and cares for her family.