Are you looking to capture the code that made the changes or just have a record of the events (e.g VM created)?
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I would capture a sequence of changes, analyze them, and potentially immediately affect additional changes. The changes are analyzed in detail before taking any action. But the objective is to do this real time as the changes occur versus after some significant time delay. The changes are also needed to maintain my representation of the system state as close to the real system's state as possible.
A SOAP proxy for VI WS seems to do the job because you can see everything and the objects are the same ones the API uses. It just creates more deployment complexity and risk.