Viral vectors are critical vehicles for the delivery of life-saving cell and gene therapies. Adeno-associated virus (AAV) and lentiviral vectors (LVV), two of the most widely used vector platforms, differ significantly in their structural properties, leading to distinct behaviours under environmental stress. However, existing analytical techniques are complex, resource-intensive and limited in their resolution and sensitivity to assess vector integrity and heterogeneity at the single-particle level. In this study, we apply mass photometry as a powerful analytical tool to characterise viral vectors following stress-testing and to assess their stability under varying process conditions. This approach provides valuable insights into vector robustness, enabling the identification of process vulnerabilities and supporting risk mitigation strategies during gene therapy development.