Together with recent technological advances in a variety of tools (such as Facebook, Twitter, Google, YouTube), learners have been provided with freedom and openness to communicate with each other and to become proactive and independent in their learning. It is believed, however, that Asian learners seem to possess reactive autonomy so that while they can organize their resources autonomously to achieve their learning goals, they are unable to take responsibility for their own learning. Therefore, the aim of this paper was to explore the factors that influence Vietnamese EFL learners' support for or resistance to promoting learner autonomy within a 15-week, portfolio-based writing course. The data were collected using semi-structured interviews and writing logs in this qualitative research and analyzed through content analysis. The findings showed three major factors (personal, academic, and external) supporting and resisting developing learner autonomy in the portfolio-based writing course.