Seismic activity in Spain is generally moderate; however, earthquakes may occasionally reach relatively high magnitudes. Social housing built during the 1960s in Andalucía is potentially vulnerable to seismic action due to inadequate construction standards and the aging structural materials. Given the social, cultural, and economic value of this building stock, assessing its seismic vulnerability is essential, particularly considering soil-structure interaction effects in buildings founded on soft soils, where seismic vibrations may be amplified. In this study, a representative building located in Polígono de San Benito (Jerez de la Frontera, Cádiz) was selected. Based on the analysis of original design drawings and available nearby geotechnical investigations, a two-dimensional soil-structure interaction model was developed using PLAXIS software. The model was calibrated using data obtained from a geophysical survey of the soil, on-site inspections employing non-destructive testing techniques (sclerometer and ultrasonic concrete test), and a simplified modal analysis of the building. Special attention was devoted to the definition of the soil strength and deformation parameters under dynamic conditions, accounting for the small-strain levels associated with seismic loading. The calibrated model was used to analyze the response of the coupled soil–structure system under free-vibration conditions and under seismic excitation using the 1995 Kozani-Grevena earthquake record, adapted in frequency. The results provide valuable insight into soil-structure interaction mechanisms. This interaction leads to the activation of different natural frequencies and to the amplification of accelerations from the foundation to the roof. Horizontal excitation mainly amplifies low-frequency components, whereas vertical excitation results in higher acceleration levels due to limited soil damping. These findings underline the need to consider multiple dynamic loading scenarios and site-specific soil conditions in seismic vulnerability assessments.