vCenter does not supply the disk trafffic such as IOPS for every VM partitioned drive (such as C:, D: etc)
From the vCenter API we can only obtain the VM traffic to it's datastore file (vmdk) via the Host vmhbax:x:x:x. If there are multiple guest OS disks partitioned in the VM datastore file, then vCenter simply aggregates their traffic.
In fact, even if one VM has multiple vmdk on the same datastore, then they are also aggregated by vCenter. This is the VM's vmhbax:blah metric seen in the MP (and I agree, it is a bit 'blah' and not user-friendly, but that's VMware's fault
In vCenter 4.1 and later VMware added the virtual disk
capability to the API metrics, so we will be able to add each virtual disk (vmdk) as a separate instance for I/O in our next update. Meaning that if a VM has 2 vmdk files, even on the same datastore, we will be able to separate the traffic for them. However vCenter still does not supply I/O metrics per VM guest partitioned disk inside the virtual disk. So that's a feature request for VMware
For the moment, you would have to check how your disks are configured for each VM - for every Datastore that a VM is using, there will be a vmhbax:x:x:x metric showing the I/O. This will combine the metrics for all partitioned guest disks on that datastore, and you can report in the MP on that.
Hope that helps!