Intervention by First Responders (FRs) is essential in disaster response, and their preparation greatly benefits from continuous updates. However, the design of effective training experiences targeted to FRs can be very demanding from the viewpoint of a Training Provisioner (TP). Virtual Reality (VR) may have a key role to play in enhancing and facilitating this task. In fact, VR technology has already proven to be very helpful in the field of emergency training, as well as its use as a powerful design and mock-up tool in many other contexts. In this work, the application of VR as a mock-up tool supporting TPs in the arrangement and validation of a training experience, either real or virtual, is explored. In particular, a case study is considered concerning the training of an FR for hydro-geological risks. Within this context, the proposed approach is compared against dramaturgy prototyping, a method commonly used for the design of experiential courses. Results indicate that the adoption of a VR-based mock-up tool (VRMT) can provide TPs with good insights on the arrangement of the training and precious indications on how to actually map this information onto real-world exercises.