Please find updates as follows:
1. My understanding is that Orion presents you a total memory used by the server as reported via SNMP.
2. What Vman (and VCenter) shows is "Memory Granted" counter. That is not necessarily equivalent of total memoru used on a host server. As per Vmware documentation:
Memory granted is a "Sum of all granted metrics for all powered-on virtual machines, plus machine memory for vSphere services on the host."