She attended and graduated from Barbizon Modeling and Acting School in Arizona.
Her most notable roles include the children"s book heroine Eloise in Eloise at the Plaza and Eloise at Christmastime, Ariel DuBois in the Emmy-winning television series Medium, and teenage cancer patient Kate Fitzgerald in the 2009 film adaptation of My Sister"s Keeper by Jodi Picoult. Less than a year later, Vassilieva received her first role, playing the granddaughter of a retired Russian General in an episode of the Columbia Broadcasting System television series The Agency. Vassilieva played Cindy Brady in The Brady Bunch in the White House, opposite Shelley Long and Gary Cole in 2002.
In 2003, she played Eloise in two made-for-television movies: Eloise at the Plaza and Eloise at Christmastime, directed by Kevin Lima and featuring Julie Andrews as Nanny.
Foreign her role as Eloise, Vassilieva was nominated for the Best Leading Young Actress in a television Movie, Miniseries or Special at the 2004 Young Artist Awards. In May 2004, at the age of eleven, Vassilieva was cast as Ariel DuBois in Medium, a series about mother-of-three and consultant to the Phoenix District Attorney"s office Allison DuBois (Patricia Arquette), whose psychic visions help solve crimes.
The series premiered on January 3, 2005. Vassilieva played the daughter of Arquette"s character for seven seasons, until the series" finale on January 21, 2011.
Vassilieva played the lead role of terminally ill teenager Kate Fitzgerald in the 2009 adaptation of Jodi Picoult"s novel My Sister"s Keeper, directed by Nick Cassavetes.
She starred alongside Cameron Diaz, Jason Patric, and Abigail Breslin. Vassilieva shaved her head and eyebrows for her role as a teenage cancer patient. In 2011, she guest-starred on Law & Order: SVU as Sarah Walsh, a piano Bachelor of Fine Arts candidate who was raped in her New York City apartment.
She reprised her role as Sarah in 2013.
On May 21, 2014, Vassilieva graduated from Columbia College, Columbia University with a Bachelor of Arts and Science degree. She has also completed a Linguistics major with honors, and a special concentration in Business Management.
Vassilieva is a member of Alpha Omicron Pi Fraternity.