Multimodal haptic rendering that simultaneously presents stimuli of multiple haptic modalities, such as vibration, impact, and thermal, has the potential to provide richer and more immersive haptic experiences than unimodal haptic rendering. For that, maintaining the synchrony between distinct haptic stimuli is crucial, as it significantly affects the overall quality of haptic sensations perceived by the user. In this paper, we investigate the perceptual sensitivity to the simultaneity between multimodal haptic stimuli combining impact and vibration. We design a handheld multimodal haptic device and conduct a perceptual experiment to measure the thresholds of perceived simultaneity between the two modalities while varying the vibration frequency and duration. Our results show that the order of two tactile stimuli for simultaneity perception is affected by both vibration frequency and duration. Specific results about the effects of vibration frequency and duration provide useful insights and design guidelines for eliciting synchronous sensations in multimodal impact and vibration rendering.