Hard-grade asphalt binders, such as AC 20–30, offer excellent resistance to permanent deformation but are inherently brittle, making them highly susceptible to fatigue and low-temperature cracking. While polymer modification addresses these issues, virgin polymers remain expensive. Despite the growing interest in recycled plastics, the rheological impact of complex waste streams, specifically polyvinyl chloride (PVC) derived from flex banners containing plasticizers, on excessively stiff binders within the complete Superpave Performance Grading (PG) framework remains critically underexplored. This study introduces a novel valorization approach by utilizing solvent-extracted flex banner waste (WPVC) as a dual-action modifier. It leverages the stiffening effect of the PVC network alongside the softening effect of leached plasticizers to redistribute the viscoelastic properties of hard-grade bitumen. The primary objective is to upgrade AC 20–30 into a balanced, climate-resilient binder. WPVC was incorporated at 10%, 20%, and 30% by weight. A comprehensive evaluation was performed using microstructural and chemical analyses, including Fourier transform infrared (FTIR), energy-dispersive X-ray (EDX), and optical microscopy, along with advanced rheological testing such as frequency sweep (master curve), multiple stress creep recovery (MSCR), linear amplitude sweep (LAS), and the Glover–Rowe (G–R) index. Results demonstrated that WPVC integration significantly improved microstructural homogeneity without accelerating oxidative aging. The 20% WPVC dosage emerged as the optimum configuration, effectively reducing rotational viscosity and excessive high-temperature stiffness while maintaining exceptional rutting resistance suitable for extreme traffic (PG82-E). Crucially, the intermediate-temperature performance was remarkably enhanced; the WPVC20 blend exhibited a 918% increase in fatigue life (at 2.5% strain) in the LAS test compared to the base binder. Furthermore, the G-R index confirmed a substantial reduction in cracking susceptibility. Ultimately, this research provides a sustainable, data-driven pathway for transforming hazardous flex banner waste into a high-value modifier for durable flexible pavements.