A domestic biogas digester (BD) is a household-sized system that produces biogas from organic waste under anaerobic conditions. By substituting conventional cooking fuel with biogas, greenhouse gas (GHG) emissions can be reduced. In addition, improved livestock manure management from use of the BD system can lead to further GHG emission reductions. However, because the main component of biogas is methane (CH4), with 25 times the global warming potential of carbon dioxide (CO2), leakage of biogas from the BD system can counteract the benefits of this system. Thick vinyl-type BDs were introduced to farming households in a rural area of Vietnam’s Mekong Delta in an effort to reduce GHG emissions as a Clean Development Mechanism project. In this project, selected farmers were trained as Key Farmers (KFs). These farmers acquired knowledge about appropriate techniques relating to the BD system and provided technical support to households. Then, biogas usage was monitored in the households where the BD technology was installed. The average proportion of operational BDs reached as high as 92.3%. Therefore, the technical support system provided by the KFs was regarded as practical and effective. Additionally, leakage of biogas from the BD systems was monitored. The average leakage was estimated at 8% of the produced biogas. Including emission reductions from improved livestock manure management, the total GHG emission reductions from the introduction of BD systems was calculated as 2.95 tonnes of CO2 per year per household. Overall, the emission reduction effects can only be achieved with appropriate installation, operation, and maintenance of the BD systems.