I don't run them side by side but my understanding is that Virtualization Manager provides a much deeper dive on the virtualization stuff including vm/host/storage mappings and such. Orion NPM is much higher level but also provides all of the network and system monitoring functionality that you won't get with Virtualization Manager. Which tool is best really depends on what problem you are trying to solve.
Byrona is exactly right. Virtualization Manager collects more data, analyzes it from a virtual point of view (alerts, trends, reports) and gives you actionable info for your virtual environment. I am happy to chat with you directly about Virtualization Manager if you have time - just contact me directly.
You can also checkout our latest Virtualization Manager - What we are working on post and see/vote on the some of the integration features we are working on.