<Tr> <Th> Years active </Th> <Td> 2003--present </Td> </Tr> <Tr> <Th> Labels </Th> <Td> Independent </Td> </Tr> <P> Olivia Rose Olson (born May 21, 1992) is an American actress and singer - songwriter, mostly known for her voice roles as Vanessa Doofenshmirtz in Phineas and Ferb and Marceline the Vampire Queen in Adventure Time . She also played the character of Joanna in the 2003 film Love Actually and its 2017 short sequel Red Nose Day Actually . </P> <P> Olson is also a writer, contributing a section to the best - seller Adventure Time Encyclopedia and an upcoming book for Cartoon Network . </P>

Who does the voice of vanessa on phineas and ferb