In our setup OVM Manager is installed on its own physical server, using for storage the local disk. Does the storage discovering process requires OVM Manager to have access to the SAN/NAS VLAN, or is this going through the OVM Servers again as it happens during the initial servers' discovering?
no, the OVMM doesn't need access to any of the storages the OMV servers are connected to. It is all done through the ovs-agents running on the OVM servers. Those query the hardware and available storage upon the discovery process.