We consider a vacuum cluster tool with intermediate buffers between the vacuum processing area and the atmospheric wafer input-output area. The intermediate buffers which temporarily store at the slots wafers for processing or processed ones should be appropriately controlled. The buffers should be pumped down to the vacuum state or vented to the atmospheric state for transferring wafers between the two areas. We examine the operating strategies of pumping and venting operations at the buffers, which are associated with the wafer handling robots in the two areas. We propose the optimal robot task sequences and the buffer operating procedure.