While NSX and vRNI are often paired, vRNI has exactly the same capabilities when you're not using NSX. You can add the physical network devices (you should also do this when using NSX, btw) and vSphere environments to ingest the data and then take advantage of the security visibility (what's talking to what, virtual & physical), networking 2-day ops by having your network topology on hand, checking for any issues there, it'll use the built-in health checks and events that react to configuration and operational (packet loss, devices that have 100% CPU, etc) and you can get notifications from those.
When NSX is added to vRNI, you'll have the same visibility in the NSX topology that you can get into the physical network. The added value here is that the virtual and physical network topologies mesh together and visible in 1 diagram. Without NSX, you still have the physical network visibility and all the other capabilities of vRNI.
Hope that helps,