Outsourcing clinical trials sponsored by pharmaceutical companies from industrialized countries to low- (middle)-income countries - summarized as transnational biomedical research (TBR) - has lead to many concerns about ethical standards. Whether study participants are particularly vulnerable is one of those concerns. However, the concept of vulnerability is still vague and varies in its definition. Despite the fact that important international ethical guidelines such as the Declaration of Helsinki by the World Medical Association or the Ethical Guidelines for Biomedical Research Involving Human Subjects by the Council of International Organizations of Medical Sciences refer to vulnerability as ethical principle, each of their approaches are different. To overcome these shortcomings, we analyze and unite different approaches of vulnerability and develop practical criteria in order to operationalize the concept especially for the context of TBR. These criteria refer to the context of a study as well as the characteristics and the current living situation of study participants. Based on a case study of an HIV-vaccine-trial conducted in India we demonstrate how those criteria can be applied in a retrospective way to identify potential ethical conflicts. The criteria can also indicate a prospective function for ethical pre-assessment. For this, we provide an outlook for three major topics: 1. Vulnerability as a normative concept: Different ways of protection; 2. The relevance of transparency and 3. Vulnerability as an instrument to increase decision participation of human subjects.