Foreign Language Anxiety (FLA) is widely recognized as an emotional factor that significantly affects students' learning and well-being. For decades, research has focused on this learner-related factor, primarily viewing it as a restraining factor and seeking to minimize or eliminate it. Most studies have employed quantitative methods and thus tend to treat all students' anxiety levels as the same. Therefore, this paper adopts a qualitative approach based on Vygotsky's sociocultural theory to explore the nature and causes of FLA in Vietnamese university students. According to this theory, classroom activities are actually social interactions, in which the main components of the teacherstudent and student-student relationships influence each other. The research results show that the main sources of FLA are teachers and peers. Additionally, FLA is flexible, varies between students, and exhibits some positive aspects. The paper also suggests that although FLA is common, it needs to be studied locally in different sociocultural contexts and environments, preferably at the individual level.