Viet Nam has made significant progress in expanding social insurance coverage in recent years. However, coverage amongst small and medium-sized enterprises (SMEs) remains very low and very few workers in this sector are expected to receive a pension in retirement. Drawing on two datasets for SMEs in Viet Nam, this paper seeks to explain this phenomenon by examining the characteristics of enterprises that are enrolled and those that opt out, and it identifies possible barriers to enrolment, such as high contribution rates. It also examines how enforcement mechanisms and formalisation policies might deter enterprises from enrolling. Drawing on lessons from international experience, the paper recommends a series of policy responses that seek both to address these barriers and to protect the livelihoods of those workers who are not yet covered.