In upper division electricity and magnetism, the manipulation and interpretation of vector functions is pervasive and a significant challenge to students. At CSU San Marcos, using in-class activities adapted from the Oregon State University Paradigms in Physics Curriculum, students' difficulties with vector functions become evident in two types of in-class activities: sketching vector functions and relating vector and scalar functions (e.g., electric field and electric potential). For many students, the cause of these difficulties is a failure to fully distinguish between the components of a vector function and its coordinate variables. To address this difficulty, we implement an additional in-class activity requiring students to translate between graphical and algebraic representations of vector functions. We present our experience with these issues, how to address them, and how in-class activities can provide evidence of student thinking that facilitates curricular refinement.