Vacuum Packed Particles Torsional Dampers (VPPTDs) offer a promising solution for adaptive vibration damping in systems requiring tunable dynamic responses. However, modeling their complex hysteretic behavior poses significant challenges. In this study, a modified Bouc-Wen model is proposed to more accurately capture the torque vs angle characteristics of VPPTDs. The modification includes an additional nonlinear stiffness component and a dry friction term, both essential for reflecting the system’s behavior under varying conditions. Quasi-static experiments were conducted on a custom-designed test stand under harmonic excitation (±10°) at frequencies ranging from 0 to 0.8 Hz and underpressures between 0 and 0.09 MPa. Tests were performed for various granular mixtures of PET-NBR particles with mass ratios of 1:0, 3:1, 1:1, 1:3, and 0:1. Shapes of measured torque-angular displacement hysteresis loops strongly depends on underpressure and mixture. Parameters of the modified Bouc-Wen model were identified using evolutionary algorithms and were found to be functions of the applied underpressure, enabling pressure-dependent tuning of stiffness and damping characteristics. Dynamic simulations using the identified models revealed the potential for effective control of the system's response by adjusting the vacuum level. The results underscore the modified model's suitability for capturing VPPTD behavior and highlight the feasibility of using underpressure as a control input for adaptive damping in mechanical systems.