An active vibration control system using an inertial actuator will be used to assure the vibration isolation of a chassis in the presence of multiple unknown time varying tonal vibrations located in two distinct regions in the frequency domain. The objective is to minimize the residual force by applying an appropriate control effort through the inertial actuator. The system does not use any additional sensor for getting in real-time information upon the disturbances. A hierarchical feedback approach will be used for the control of the system. At the first level a robust linear controller will be designed taking advantage of the knowledge of the domains of variation of the frequencies of the vibrations. However the performance of the linear controller is somehow limited because of the width of the regions of variation of the frequencies of the vibrations and the robustness constraints. To further improve the performance, a direct adaptive control algorithm will be added. Its design takes into account the internal model principle for disturbance rejection and is conveniently implemented through the Youla-Kučera parametrization of the controller. Experimental results obtained on a relevant test bench will illustrate the methodology.