Walden was currently involved with another girl (Kate) at the time, but became almost instantly enamored of Vivian, who at the time was hiking and asked if she could refill her canteen. Since there was a storm outside, Walden invites her to stay the night and they have Chinese food, bonding over their ability to instantly come up with bumper-sticker statements. Vivian describes her traveling experiences, and Walden daydreams about sailing around the world with her. However, she remains (initially) oblivious to his attractions. The next morning, Vivian thanks him for his hospitality and leaves.
After jilting Kate because he thought Vivian was 'the One' (resulting in Kate kicking him in the balls) Walden later followed her and found her setting up her tent in the woods; he startled her, resulting in her kicking him in the balls too. Walden confesses his feelings for her and offers to fly her anywhere she'd like to go, but Vivian (while not unsympathetic) points out that he is the type of guy who falls in love very easily- and, while Walden is thinking of the destination, for her the journey is the destination. She sends Walden home. Later, Walden daydreams that Vivian and Kate both come back to him and have a threesome with him.
Vivan says her parents named her after a character on "that 70's show...Maude" (a clever nod to the show on which Ashton Kutcher and Kunis met and worked together).
Ironically, Vivian is played by Mila Kunis, Kutcher's real-life fiance and mother of his daughter.