SCM systems are platforms that enable companies to buy efficiently, both directly from vendors and through intermediaries like exchanges. The best known systems are provided by SAP, Oracle, i2, Baan, etc. A write-up here: http://en.wikipedia.org/wiki/Supply_chain_management_software. An examination of the SCM functionality is a great place to ideate powerful features for the VRM toolkit.

VRM is SCM for individuals. Or, perhaps more accurately, traditional SCM is the sub-species of VRM that is applied to organizations.