Both cloud server and cloud VPS are running on the same virtualized environment, this basically means that resources like CPU and memory come from the same hypervisor.
"Cloud VPS" is different in that total resources for all VPS's is set to one overall account. Also, the OS templates available are usually restricted for VPS.
While "Cloud Server" is different in that the resources available are only restricted by the plan ordered by the customer. That plan can be changed to allow more if needed. There are more OS templates to select from. Also, it should be noted that more than one hypervisor can be used.
So, "Cloud Server" versus "Cloud VPS" is trying to define how much flexibility a user has. "Cloud VPS" is shown as restricted and "Cloud Server" as more flexible.