Fluency development is one of the prominent components of a well-balanced language course. In particular, speaking fluency interests second language (L2) researchers who have continuously tried to look for the finest approach for helping L2 learners attain a certain level of fluency to achieve effective communication. To this end, the primary purpose of this research is to explore what factors affect English-speaking fluency and what strategies can effectively boost its development among a cohort of Vietnamese university students. The participants were 142 English majors who filled out a questionnaire measuring their perceived self-efficacy concerning English-speaking fluency. Four teachers and six students joined the semi-structured interviews. Quantitative and qualitative data revealed that the most influential factors were linguistic elements, followed by performance, and affective factors. The most effective strategy for enhancing fluency was task repetition. Notably, the findings revealed a mismatch between teachers' and students' understandings of speaking fluency, which may negatively impact the achievement of fluent speech. Based on these results, pedagogical implications are discussed for English teachers and students regarding fluency development.