What's the Database you're using (built-in SQL express/ separate DB) ...?
In SQL you could querry the VPX_TASK
Something like this:
select task_id, parent_task_id, start_time, username, complete_time, entity_name, descriptionid, complete_state
from VPX_TASK
This used to work in vCenter 5.0 with SQL ...
Since I don't have access to my lab at the moment, I can't really test much.