The difference is the license which comes with the purchase.
VSX is a software feature. It has nothing to do with the hardware. It works on open servers or appliances (which are just really expensive and non-standard open servers). Last I checked, VSX can even be enabled without a specific license and it will let you run one VS this way for separation of management traffic from through-traffic. Running more VSs takes a license for the feature.
There is also nothing virtual about it beyond the virtualization inherent in preemptive multitasking. VSs are just additional routing tables (VRFs in old software versions, network namespaces in GAiA 3.10). There is exactly as much separation between VSs as there is between Cisco VRFs or Docker containers on Linux. Less separation than there is between Cisco VDCs, FreeBSD jails, or Solaris zones. It's all one single OS.