NoMachine doesn’t use VNC or Xvnc to share the session. It uses its own system. There is a way to create a NX session as a connection to a remote VNC server, but this is not what you are looking for.
Normally NoMachine remotes the display :0. You can change the display to your Xvnc display as explained in this article (which uses Xfvb as an example) https://www.nomachine.com/AR10K00710.
To set which display will be shared with change the node.cfg key: #DisplayDefault :0