Hi,
A major component of VRF security is path isolation. As the guide below states this isolation means that, "no communication between devices belonging to different VRFs is allowed unless explicitly configured."
http://www.cisco.com/en/US/docs/solutions/Enterprise/Network_Virtualization/PathIsol.html
However I would not rely upon it as your only security measure within your network.
The guide also discusses how to deploy this technology behind your firewall. Another excellent reference is the Cisco Validated Design for a Virtualised Multi-Tenant Data Centre that can be found here:
http://www.cisco.com/en/US/docs/solutions/Enterprise/Data_Center/VMDC/2.2/design_guide/vmdcDesign22.html
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