This case study explores the Viability Gap Funding (VGF) financing instrument provided by the Technical Assistance Facility (TAF) to the Private Infrastructure Development Group (PIDG) project development and financing facilities. VGF helps PIDG facilities offset part of the up-front preparation costs of pro-poor private infrastructure investments in challenging environments that prevent projects towards attracting debt financing and hence reaching financial close. To illustrate how VGF is operationalized this case study analyses VGF provided by TAF to the InfraCo Asia Development (IAD) supported project of Coc San hydropower plant in Vietnam. This case study illustrates key challenges and opportunities of PIDG facilities using TAF VGF funding associated with the complexity and high risk nature of the instrument.