VxRail delivers a ready environment with vSAN with multiple networks that you have defined during startup.
Can I make changes to the networks, how to create new ones or change existing ones?
You can add more VM networks and edit or delete these additional networks/portgroups. But it is not supported to change or delete the initially created management networks yourself. You have to contact the Dell EMC Support for this. This includes the vSAN, vMotion, Marvin and VMware Server network.