 As the landing gear was being lowered on approach to Portland International Airport (PDX), the crew felt an abnormal vibration and yaw of the aircraft as well as a lack of an indicator light showing the gear was lowered successfully.The crew requested a holding pattern to diagnose the problem, and for approximately the next hour the crew flew over southeast Portland and worked to identify the status of the landing gear and prepare for a potential emergency landing.During this time, none of the three cockpit flight crew effectively monitored the fuel levels, which was exacerbated by the fact that the gear was down with the flaps at 15 degrees during the entire hour-long holding maneuver, significantly increasing fuel burn rate.