A semi-active torsional vibration absorber integrating quasi-zero-stiffness (QZS) vibration isolation and variable-stiffness vibration absorption is proposed for vibration control in vehicle transmission systems. The QZS vibration isolation module consists of a linkage-disc spring negative stiffness mechanism connected in parallel with positive-stiffness cylindrical springs, while the vibration absorption module is designed based on magnetorheological elastomers (MREs). The structures of both modules are introduced, and models are established to analyze their characteristics. A simplified vehicle transmission system is constructed to simulate the vibration reduction performance of the semi-active torsional vibration absorber under uniform variable-speed conditions. The results show that the semi-active integrated torsional vibration absorber can effectively isolate low-frequency vibrations while tracking the system's frequency to achieve efficient vibration control. It combines both vibration isolation and absorption characteristics, demonstrating significantly better vibration reduction performance than passive vibration absorbers.