The aims of the paper are to describe the Vietnamese offshore software outsourcing industry and to investigate key success factors of the largest offshore software outsourcing company in Vietnam, FPT Software. Vietnam has taken ICT industry a strategic one for its economic development, and it has been showing high growth in that field. Especially, Vietnam has potential to be successful in offshore software outsourcing, but Vietnam won't be sustainable in this competitive market unless it overcomes several weaknesses. FPT Software as the best achiever so far is a good model for Vietnamese software companies. The key success factors of sustainable development in FPT Software are found in good managerial practices, application of international quality standards for software development, and highly motivated and skilled workforces. Based on the case study, we discuss the sustainability of Vietnam's offshore software outsourcing.