During a droplet impact onto a substrate, splashing is known to be caused by the presence of surrounding gas or by surface roughness. Impact occurring in a vacuum onto a smooth rigid wall results in droplet spreading, rather than development of a corona or prompt splash. Here we present an analytical and numerical study of a third potential splashing mechanism, namely elastic deformation of the substrate. An axisymmetric Wagner-style model of droplet impact is formulated and solved using the method of normal modes, together with asymptotic analysis and numerical methods. We highlight the effect that a flexible substrate brings to the contact line velocity and jet behaviour, demonstrating that oscillation of the substrate can cause blow-up of the splash jet which is absent for a rigid substrate and indicate the onset of splashing.