I am wondering how to kick off a workflow in Orchestrator when an Action is performed on a VM. For instance, if a user selects the destroy action on a VM a workflow in Orchestrator is executed.
Yes, this is possible using the Event Broker Service. When the Destroy action is called, vRA will run through a series of internal workflow states, from which you can call a vRO workflow.
Have a look here for details on how EBS works:
Exploring the vRealize Automation 7.0 Event Broker – Part 1 | The Virtual Viking
Ah, yes. My bad. The equivalent functionality in v6 is known as Extensibility.
Have a read here, the basic functionality is the same, it just gets configured in a different way.