A simple and rapid approach to analyse the failure hazard of historical masonry vaults is presented, using a paradigmatic case study from a massive urban aggregate in Piacenza (Emilia Romagna, Italy). The curved structure under examination is a cloister vault with a considerable span, over which a wall was constructed in the key position. The urgency of a stability assessment to prevent failure is evident. The geometrical characteristics and some material information were obtained through in situ surveys. To assess its stability and determine the risk of failure a 2D finite element model is proposed, in which the bricks are discretized using 4-noded elastic elements in plane stress, and the mortar joints are characterized by (i) orthotropic shell elements coupled with (ii) ductile cutoff bars. The backfill is considered in the model by using a system of equivalent forces reproducing both soil self-weight and horizontal earth pressure. Additionally, a possible retrofitting intervention using Carbon Fiber Reinforced Polymer (CFRP) is tested and it is modelled with elastic perfectly ductile cutoff bars placed at the intrados of the vault and connected to each node of the mesh, ensuring a perfect bond between substrate and reinforcement. After a thorough analysis of the results, including force-displacement curves and a detailed examination of the failure mechanisms, an assessment of the cloister vault is provided.