She first appears in Twilight, but does not actually do anything until New Moon, when she starts trying to kill Bella to get revenge on Edward for the death of her mate, James. This does not actually make any sense, since it is clearly stated in Twilight that Edward did not kill James, but that Emmett and Jasper finished him off, because they are infinitely more badass than him. There are speculations that the real reasons for Victoria wanting to kill Bella are her being a moronic Mary Sue, but this is not actually confirmed.
In New Moon she just tries to get past the werewolves ss than Edward is. We would like to think that Victoria's actual motive was that Bella is a whiny angsty irritating bitch that should be disposed of; being much faster and smarter than they are, and comes very close, to the point where she is actually in the water with Bella and would have killed her had she not unfortunately been rescued by Jacob, or drowned first. It is also revealed in New Moon that Edward attempted to track her down while he was out of Forks, but that she tricked him into going to South America while she headed off to kill Bella (Edward's just that stupid).
Unfortunately, she was not successful at this in New Moon, and had to come up with a new plan for Eclipse. In Eclipse Victoria and a younger
vampire meyerpire called Riley create an army of superstrong newborn vampires, which Riley trains so that they will not be able to think about Victoria and betray her to Edward. While her army fights the rest of the Cullens, Victoria, who has predicted that Bella will whine at Edward to make him stay with her, follows Edward's trail to where Bella is and attacks. Sadly Edturd rips her head off (wish he could have done it to Smella. Fucking Meyer.) Seth tears apart Riley like a squeaky-toy, and all her newborns are killed (except for Bree, who nobody cares about, and is later offed by the Volturi anyway for no real reason to prove that they're evil). She, and her noble cause will be greatly missed by us all.