The assessment of the seismic vulnerability of masonry buildings in aggregate typical of historic centers is a highly complex and widely debated topic. A review of the current state of the art reveals several critical and open issues. The aim of this work is to contribute to the modeling of the interaction between structural units (SUs) and the so called “aggregate effect”. Given the complexity of the theme, this paper focuses on analyzing an aggregate with a regular plan configuration, allowing for the investigation of the in-plane (IP) response of both the entire aggregate and individual SUs, without the influence of potential torsional effect associated with plan irregularity. The aggregate selected for the analysis is a row aggregate located in Mirandola (MO), an area that was affected by the seismic swarm of May 2012. The wall panels of the SUs are made of solid bricks with variable thickness, while the floors, predominantly wooden, are arranged parallel or perpendicular to the street side. A 3D numerical model was performed according to the equivalent frame modeling strategy with the Tremuri software. Special attention was given to evaluating the historical evolution of the aggregate in order to accurately model the boundary conditions of each SU. Two different modeling criteria are proposed, depending on the interaction between the SUs. Through nonlinear dynamic analyses (NLDAs), fragility curves for IP vulnerability were derived for both the entire aggregate and some SUs in their isolated configurations.