Using classroom observations (formative) and student course experience survey results (summative) to evaluate English lecturers’ teaching performances is not new in practice, but surprisingly only a few studies have investigated this issue in a higher education context. This study was conducted in an English department of a large university in Vietnam. The data include: (1) semi-structured interviews with all the full-time lecturers, (2) two department heads and (3) course experience surveys from English as a foreign language (EFL) students (N = 2886). Three lessons can be learned: (1) formative assessments do not seem to have an effect on promoting better teaching practices when their feedback is not helpful in improving high-stakes summative assessment results, (2) without sharing a common definition of good teaching practices among assessors, summative assessments appear to make the feedback from formative assessments less meaningful and applicable, and (3) as a result, the combination of formative and summative assessments tends to make EFL lecturers’ self-assessment practices less effective.