Teachers in Vietnam need professional learning in order to meet the national requirements of comprehensive educational reform. Teacher professional development encompasses all forms of continuous education for educators. Teacher professional learning is a continual process that begins with schools determining the educational outcomes that are crucial to their pupils. This study aimed to understand the status of teacher participation in the two professional learning forms (formal & informal) and the influence of these on professional development. The paper is built on the data collected from a survey conducted at high schools from four provinces in Vietnam in 2020. Responses to a paper questionnaire were provided by 355 teachers. The investigation results show, firstly, that the majority of teachers developed their expertise through informal pathways such as teachers actively participating in professional activities at school and actively learning, sharing and exchanging professional experiences with colleagues at school. In contrast, teachers did not participate much in formal forms, and the number of participants in scientific research and certificate and rank promotion courses was the least. Moreover, while formal professional learning forms had a positive influence on the professional development of teachers, informal forms had a higher degree of influence.