To study the influence of L-shaped flow deflectors on the vortex-induced vibration (VIV) characteristics of steel box girders, a series of 20 test cases was designed. A sectional model wind tunnel test was carried out under a +5° wind attack angle to analyze the VIV response patterns associated with variations in horizontal plate width and vertical plate height. In parallel, computational fluid dynamics (CFD) simulations were performed, and the vortex structures were extracted using the Ω vortex identification method to reveal the underlying VIV evolution mechanism. The results show that increasing the horizontal plate width significantly enhances the suppression of vertical bending VIV, while shifting the VIV lock-in region toward higher wind speeds. In contrast, the vertical plate height strongly influences torsional VIV; larger heights tend to induce torsional vibration in the high wind speed regime. Flow field analysis shows that widening the horizontal plate reduces the spanwise extent of vortex structures, thereby improving flow uniformity. Conversely, increasing the vertical plate height promotes the formation and development of vortex clusters, leading to a more complex flow field. Effective vibration mitigation is achieved only when the vortices generated by the attached components are of a comparable scale to those shed from the main girder, enabling interference with the dominant vortex-shedding process.