Abstract: Achieving a robust strength–ductility synergy remains a central challenge in advanced steels. Here, vanadium (V) microalloying was used to overcome the strength–ductility trade-off in a medium-carbon low-alloy steel subjected to flash heating, achieving a tensile strength of 2246 MPa and a total elongation of 15.3%. Compared with the V-free steel, V addition significantly refines the martensitic hierarchical structure, reducing the prior austenite grain size from 9.62 to 4.30 μm and the martensite block size from 1.66 to 0.86 μm. Electron backscatter diffraction (EBSD) and parent-grain reconstruction show that this refinement promotes Closely Packed (CP) group biased variant selection and increases the density of high-angle grain boundaries (HAGBs) from 1302.7 to 2318.1 mm⁻¹. As a result, post-uniform elongation (PUE) increases from 6.2% to 13.0%, while reduction of area (RoA) rises from 49.4% to 56.9%. X-ray microtomography and post-fracture SEM reveal more tortuous crack paths and abundant secondary microcracks in the V-microalloyed steel, consistent with enhanced crack deflection by the dense HAGB network. These results demonstrate that the synergy between V microalloying and flash heating provides a practical and industrially scalable route for developing ultra-high-strength steels with improved ductility.