Integration of substance use disorder (SUD) treatment and HIV care can increase antiretroviral therapy coverage among people with opioid use disorder (OUD). However, implementation of integrated treatment models remains limited. Stigma towards people with OUD poses a barrier to initiation of, and adherence to, HIV treatment. We sought to understand the extent of stigma towards SUD and HIV among people with OUD in Vietnam, and the effect of stigma on integrated OUD and HIV treatment services utilization. Between 2013 and 2015, we conducted in-depth interviews with 43 patients and 43 providers at 7 methadone clinics and 8 HIV clinics across 4 provinces in Vietnam. We used thematic analysis with a mixed deductive and inductive approach at the semantic level to analyze key topics. Two main themes were identified: (1) Confidentiality concerns about HIV status make patients reluctant to receive integrated care at HIV clinics, given the requirements for daily buprenorphine dosing at HIV clinics. (2) Provider stigma existed mostly toward people with OUD and seemed to center on the belief that substance use causes a deterioration in one’s morals, and was most frequently manifested in the form of providers’ apprehensive approach towards patients. Concerns regarding stigmatization may cause patients to feel reluctant to receive treatment for both OUD and HIV at a single integrated clinic. Interventions to reduce stigma at the clinic and policy levels may thus serve to improve initiation of and adherence to integrated care.