Whether being guided by business, philanthropy or humanitarian objectives, design approaches for low income population try to propose innovative solutions with a sustainable logic. One strategy developed in the literature is the frugal engineering approach that simplifies the features of the product, lower the cost while keeping the quality high. The first step of designing for the Base of the socioeconomic Pyramid is to understand the local context and users' needs. In this paper, we aim to understand the "local intelligences", indicators of adaptations and appropriation, which are part of the product (re)design. Second, we focus on understanding how to formulize this complex new knowledge - complex because driven from ethnographic methods and carrying a lot of impactful socio-technical dimensions - for frugal design. Our hypothesis is that Personas (specific and concrete model of the users) are likely to carry representation of the users and several dimensions of the ecosystem, and thus help to pilot frugal design. To answer to our goals, we conducted ethnographic studies on six rehabilitation centers in Vietnam to gather data on design, manufacturing process, and uses of low cost prosthesis for poor amputees. The results show the mechanisms locally implemented to provide frugal prosthesis for Vietnamese amputees. The implications are for both managerial practitioners and researchers to ask the good questions on how to make knowledge more representable and transferable in a research-design collaboration.