What I would say is that it would generally be far easier to maintain iSCSI on a standard vSwitch.
You shouldn't generally be sharing iSCSI with other network services, so VLAN tags shouldn't be required or regularly changing. This wouldn't be a situation you would gain from a vDS - so I would keep it simple and not do it.
But to answer your question, vCenter is required to change a vDS, it is not required to enable it. Your iSCSI should operate fine with the vCenter on a vDS. Except that you could make things very difficult if you accidentally broke that vDS and couldn't repair it without vCenter.
I see your point in keeping it simple with a vSS. In this case it would be a server with 2x10gb links and 2x1000mb links.
Would the optimal setup be vCenter and the storage for vCenter connected to a vSS and the rest of the VM's and storage for thoes VM's connected to a dVS?