And if you don't want to see which VM's are not monitored but literally when a single VM is on/off network then you would simply add that node to be monitored then copy the "Alert me when a node goes down" alert - and add a simple condition and pick from the drop down menus Network Nodes/Node Details/NodeID and then is equal to and your nodeID for that specific device. Rename the alert something that makes sense to you and add whatever actions you want. Done.
If you need to know the node ID it's on the node details page URL - as in
http://orionserver/Orion/NetPerfMon/NodeDetails.aspx?NetObject=N%3a731
where the node id is 731.
Enjoy