Standard practice in seismic fragility analysis is to rely on a single (scalar) intensity measure (IM), typically horizontal peak ground acceleration (PGA). The present paper discusses the benefits of using alternative IM as well as fragility functions based on more than one IM, i.e. vector-valued IM. A case study dealing with the seismic fragility of fuel assembly spacer grids is presented. The efficiency of various IM is compared. Uncertainties associated with scalar-IM fragility curves and vector-IM fragility functions are also compared and discussed. The question whether vector-valued fragility functions lead to a reduction of uncertainty depends on the IM. In the present case study, a fragility surface based on PGA and PGA/PGV is associated with less uncertainties compared to the fragility curve based on PGA only.