SDN and virtual networks are different tools for different use cases.
In traditional networking setups, each networking device has its own control plane and must communicate with other control planes via protocols such as [BGP](https://en.wikipedia.org/wiki/Border_Gateway_Protocol) or [OSPF](https://en.wikipedia.org/wiki/Open_Shortest_Path_First). SDN replaces these individual control planes with a centralized control plane, which enables the fleet to act as one cohesive whole, rather than multiple autonomous systems that happen to be configured to work together.
Network virtualization, on the other hand, allows you to nest more networks together — but each of those networks still must be configured with multiple autonomous control planes.