This study aims to investigate Vietnamese students' perceptions of portfolio assessment and how it affects their writing performance. Furthermore, the study explores problems encountered during the assessment process, which could offer professional support for teachers, particularly novice teachers, when applying this formative assessment to teaching writing. Data was collected through students' written papers, observations, and semi-structured interviews with a total of thirteen low- and intermediate-level undergraduate students. The findings demonstrate the aspects of the assessment usefulness, namely validity, authenticity, interactive-ness, and impact. Accordingly, the assessment provides students with a comprehensive understanding of academic writing that enhances their writing abilities. In particular, students recognize their common grammatical errors, pay more attention to organizing ideas logically, and develop writing habits throughout the writing process, which have not been carefully addressed in prior education. Nevertheless, there are a number of issues that need to be carefully considered, including idea development, plagiarism knowledge, self- and peer-assessment, and the order of writing steps. In order to improve the reliability and practicality of the assessment, solutions to those concerns are provided.