A vessel harvesting system removes a target vessel from a patient for use as a bypass. An elongated harvesting instrument inserts into a body along a path of a target vessel which includes at least one side branch. The harvesting instrument includes a cutter for applying thermal energy to sever and cauterize the side branch. An endoscopic camera captures visible-light images from a distal tip of the instrument within a dissected tunnel around the target vessel. A thermal camera captures thermograms coinciding with the visible-light images to characterize a temperature present at respective surfaces in the tunnel. An image processor (e.g., an electronic controller) renders a video stream including the visible-light images and an overlay depicting the temperatures present on at least some of the respective surfaces when applying the thermal energy. A display presenting the video stream and overlay to a user can be an augmented-reality display.