Fused deposition modeling (FDM) generates substantial material waste in the form of disposable support structures, undermining the sustainability claims of additive manufacturing when petroleum-based polymers such as acrylonitrile butadiene styrene (ABS) are used without recovery strategies. This study addresses this challenge by proposing a closed-loop recycling approach for ABS support-structure waste using a low-energy Vertical Compounding Extrusion (VCE) system integrated with a passive vibratory feeder. The system leverages gravity-assisted feeding and synchronized passive vibration to ensure stable material flow and controlled short-glass-fiber reinforcement during reprocessing. A mathematical framework based on a modified rule of mixtures is developed to model the tensile performance of recycled and reinforced filament, explicitly accounting for fiber volume fraction, length efficiency, and orientation induced by extensional flow during extrusion. Numerical simulations demonstrate a non-linear increase in composite tensile strength with increasing fiber content, indicating that controlled reinforcement can not only offset polymer degradation from thermal recycling but also enable material upcycling beyond the properties of degraded ABS. Sustainability performance is evaluated using a Resource Recovery Efficiency (RRE) metric, with simulated scenarios yielding values exceeding 93%, indicating minimal material loss. The results demonstrate that targeted recycling of waste from homogeneous FDM support structures, combined with passive, low-energy feeding and reinforcement strategies, offers a viable pathway to improve material efficiency and integrate circular economy principles with decentralized additive manufacturing systems.