Since this thread is pretty old you've probably found your answer but I beat my head against this for hours and finally found a reference on the VMWare site that mentioned in order to create a scheduled task you would call the ScheduledTaskManager.CreateScheduledTask method. Online Documentation - VMware vSphere Web Services SDK Programming Guide - VMware {code}.
Remove the word Object and change the last line to $scheduledTaskManager.CreateScheduledTask($vm.ExtensionData.MoRef,$spec)
