The standby VM is used for the SVmotion tests. When the test is started the standby VM must be on the original datastore, which in your case would be the vsan datastore.
The parameter "SVmotion/SVMotionLUNs" in the VMMark3.properties file should be set to you iSCSI datastore.
Could you post the contents of the VC-StorageVMotionSetup.txt file?
The XVMotion workload has similar requirements, but uses the DS3WebA VM.
after migrating the sandyby0 and ds3weba0 VMs to the second shared storage(in my case is the iSCSI),
and correcting the parameters: SVmotion/SVMotionLUNs, XVmotion/XVMotionLUNS, Deploy/DeployLUNs to my primary shared storage;
the test process now has no more errors about the storage!