Masonry cross vaults in historic churches represent structurally significant yet highly vulnerable elements under seismic loading, due to their complex geometry and limited capacity to accommodate horizontal displacements. This study presents an experimental investigation into the diaphragmatic behavior of such vaults when subjected to seismic actions. Taking inspiration from the transept vaults of the medieval Church of San Michele Arcangelo in Casertavecchia (Caserta, Italy), a full-scale prototype was constructed and prepared for laboratory testing, involving horizontal displacements applied to one of its base sides. The paper first describes the construction process of the specimen, including the definition of its geometry and the results of material characterization tests. This is followed by a detailed account of the test setup, with particular attention to the boundary conditions and the assumptions made to simulate in-situ constraints. Finally, a preliminary Finite Element model of the experiment is introduced, aimed at predicting the structural response and evaluating the expected strength.