After finishing gymnasium-b in 's-Hertogenbosch in 1975, Vos went on to study biology at the University of Wageningen, graduating in 1983. During her study she was active in the students movement, the peace movement and the campaign against nuclear energy. In 1984 she worked as teacher and campaigner for an environmental organisation, 'Milieudefensie' ('Defense of the Environment'). In 1990 she began to teach ecology at the Centre of Ecology of the University of Leiden until 1992. In 1989 she became a member of the GreenLeft and almost immediately entered its temporary board, that would oversee the merger of the four parties that would form the GreenLeft. She was elected party chairperson in 1990. In 1992 this office became a paid function and she left the university.