For decades, polyvinylbutyral (PVB) has been the polymer of choice to improve the impact resistance of laminated glass. PVB presents large rupture strain, large tensile strength, and excellent dissipative properties. Here, we investigate the relation between the macromolecular structure and mechanical properties of plasticized PVB by combining large strain tensile experiments with calorimetry, X-ray, and birefringence measurements. We find that the mechanical response is dominated by creep and that this plastic-like flow can effectively be described by a strain-dependent viscosity. The commonalities with other toughened polymeric materials and especially the role played by phase-separated domains with weaker physical bonds are outlined. These results could help optimize polymer design for toughness and impact applications.