Vicky's Diary is seen in A Bad Case of Diary-Uh!. It is a purple journal where Vicky writes her deepest, most darkest secrets in.
One day, Vicky uses a truth serum on Timmy to make him blurt out his secrets--he is disgusted by bare feet, clowns scare him, he is allergic to oranges, and his fish are fairy godparents. Luckily, Vicky didn't seem to hear the last part and the next day at school, she and all the students are celebrating "Bare Feet Orange-Juggling Day." Seeing that Timmy wants to exploit Vicky's secrets like she did with him, Cosmo says that a girl usually writes her secrets in a diary, so Timmy wishes for Vicky's diary so he can use her secrets to make her life miserable. Cosmo and Wanda try to stop him from reading it and inform him that a girl's diary is special place for her to write all about one's troubles, and nobody has the right to read it. However, Timmy thinks they're just being paranoid and reads an entry aloud about Wanda in squirrel-form looking fat. Deeply insulted, Wanda reads through the diary as well, becoming thirsty for revenge like Timmy. The diary also reveals that she thinks the squirrel that looked like Cosmo was cute, complete with a drawing of squirrel Cosmo wearing his shirt and tie.
Trying to manipulate Vicky's diary secrets to get Vicky to embarrass herself in front of her crush Winston Dunsworth, Timmy fails several times, actually making Winston like her even more. Finally, when Vicky becomes a cheerleader for Winston, Timmy wishes for her to blurt out all the secrets in her diary, including her tendency to lie and cheat, her hatred of cows, and how she puts her boogers in other people's history books. Winston admits that he does not mind the lying or cheating, but he loves cows and hates snot in history books and dumps Vicky.
When Timmy finally got Vicky to spill her deepest darkest secrets, he suddenly decides Cosmo was right - he had gone too far this time and decides to make it up to Vicky by trying to get Winston Dunsworth back together with Vicky. Cosmo then suggests they read Winston's diary, where they find out that he is afraid of the color pink. Since Wanda has pink hair, she transforms into a squirrel and stands in front of Winston, and Vicky beats up squirrel Wanda to defend Winston and win him back. When Timmy reveals to Vicky that he knows she didn't like the pink squirrel, she uses the truth serum to make him reveal that he read her diary, which enrages her and makes her beat him up like she did with Wanda.