VDC's - Virtual Device Contexts. Means I can split a physical nexus in to logical nexus which are totally isolated. If I want to connect VDC's together from within the same chassis there would be a cable coming out of one port on the nexus belonging to VDC 1 and physically plugging back to the same chassis to a different VDC e.g. VDC 2.
VPC - Virtual Port Channel. Its a means to provide an end host or switch(s) with multiple physical links to from two separate Nexus devices, be it the 7Ks or the 5Ks, that is one virtual link, like port-channel or etherchannel. But is not a stacking technology for Nexus.
VSS - Virtual switching system. This is a great technology whereby two physical chassis like c6500's can form in to one virtual device - this is like a stacking technology, you have one of the chassis active in the control plane, but both are forwarding data plane traffic.
The Cisco ASR 9000 is probably more of a service provider router - very powerful for routing and extremely controlled! OS and command line is a lot different that normal IOS or NXOS.
The Cisco Nexus is more a Data Center or Core network switch where it has a fairly rich feature set to support data center applications with technologies like OTV inbuilt.
Hope this helps.
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