Many robotic mechanisms rely on rotating linear (prismatic) actuators, whose performance and accuracy can be significantly affected by lateral vibration. In this study, the vibrational behavior of a flexible rotating prismatic actuator and its impact on the end-effector accuracy of a 6UPS Stewart robot are investigated. First, an analytical model is developed using the assumed modes method to evaluate the lateral vibration of a single actuator, and its results are compared with a finite element (FEM) analysis in ABAQUS. The close agreement between the two approaches confirms the validity of the numerical model. Next, the entire 6UPS Stewart robot is simulated with elastic actuators, and the resulting trajectory of the end effector (or center of mass of the payload) is compared against the corresponding rigid-robot motion. The analyses show that actuator flexibility and vibration can introduce noticeable deviations; reaching up to 6% in the end-effector’s position; highlighting the need to account for vibrational effects in the design and control of high-precision parallel robots.