You can retrieve this information from vCenter Event Manager.
Here is sample code that enumerates all poweroff events of a given virtual machine ('vm' is the input parameter of type VC:VirtualMachine):
var spec = new VcEventFilterSpec();
spec.eventTypeId = ["VmPoweredOffEvent"];
spec.entity = new VcEventFilterSpecByEntity();
spec.entity.entity = vm;
spec.entity.recursion = VcEventFilterSpecRecursionOption.self;
var events = vm.sdkConnection.eventManager.queryEvents(spec);
for each (var ev in events) {
System.log("Powered off on: " + ev.createdTime + " by: " + ev.userName);
}