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 berated 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 for two months. She is aware that her mother is just as evil as her father, since Charlene had kidnapped several of O.W.C.A.'s agents and turned them into cyborgs during Heinz's reign, similar to how Heinz defeated Perry the Platypus and turned him into Platyborg. Vanessa also might be aware that her parents had pretended to be divorced because of some financial advantages (such as junk yard sales, separate vacation trips, and coupon mail). As she leaves to go for a movie with Tony, little did she know that The Resistance is coming by to the penthouse to take her mother into custody by bringing in her captive father along. During the battle between the Resistance and Charlene's cyborgs, the Resistance manages 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 fighting. She then convinces Candace to let her have a moment with her family. However, this was all a trick that Vanessa (who is proven to be evil as her parents) made up to help both her parents and Pandaborg escape in a hover car driven by Tony. As the notorious Doofenshmirtz family escape, Vanessa introduces Tony to Heinz and Charlene; the former recognizing 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.
However, in spite of her lack of interest of doing evil, Vanessa did prove that she may be as evil and intelligent as her parents, perhaps even more sinister than her original counterpart. To a small yet significant extent, Vanessa planned a successful escape for her family by having Tony to drive them away in a hover car in case they would be put away into custody. She is also very manipulative, as she managed to deceive Candace into having a moment with her parents, which contributed to their escape, much to Candace's anger. This also might indicate that Vanessa still loves and cares for her family so much.